Cadmium Substitution - garteur

GARTEUR LIMITED 

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GARTEUR SM/AG17 TP128


Authorisation 

Prepared by 

Dr Chris J E Smith 

Title 

Technology Chief, Corrosion Control 

Date 06/11/2000 

Location 

A7 Building, QinetiQ Farnborough, Hampshire GU14 

0LX 

Co-authors 

Name 

Location 

Name 

Location 

Name 

Location 

Names 

Location 

Name 

Location 

F Andrews 

Short Brothers plc, P.O. Box 241, Belfast BT3 9DZ, UK 

K R Baldwin 

QinetiQ Farnborough, Hampshire GU14 0LX, UK 

C Brindle 

BAE Systems, Warton, Preston, Lancs PR4 1AX, UK 

E Hultgren and L Magnusson 

SAAB, Linkoping, SWEDEN 

D Marchandise 

Aerospatiale, F-92152 Suresnes Cedex, FRANCE 

Name 

E Kock 

Location Daimler Benz Aerospace Airbus, D-2800 Bremen 1 

GERMANY 

Name 

Location 

Name 

Location 

W t'Hart 

NLR (NOP), NL-8300 AD Emmeloord, 

The NETHERLANDS 

G Vaessen 

Formerly Fokker Aircraft 

Now at Sergem bv, laan van Ypenburg 150, 

NL-2289 DV Rÿswÿk (ZH) 

The NETHERLANDS 

GARTEUR SM/AG17 TP128 

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Abstract 

A series of commercially produced aluminium and zinc based coatings have been 

evaluated as potential replacements for cadmium plating for use on aerospace 

components and fasteners manufactured from steel. Coatings examined included 

electrodeposited zinc-nickel and zinc-cobalt iron, aluminium coatings produced by ion 

vapour deposition and electrodeposition from an organic bath and metallic-ceramic 

coatings containing zinc and aluminium flakes. An experimental aluminium - magnesium 

coating produced by unbalanced magnetron sputtering was also included in the 

programme. Tests have been conducted to establish the corrosion resistance and 

tribological properties of the coatings and to compare their resistance to aircraft fluids 

with cadmium plating. Studies have been made of the effects of the coatings on fatigue 

performance and stress corrosion cracking resistance of a high strength steel. Other 

coating properties examined include paint adhesion, electrical conductivity and galvanic 

compatibility with aerospace aluminium alloys. The general conclusions are that none of 

the coatings examined match the overall performance of cadmium plating. Estimates of 

the relative costs of the coatings have been made. An environmental survey has also 

been undertaken to compare the potential risks to workers involved in the manufacture 

and maintenance of aircraft and to assess their impact on the environment. Areas for 

future research are identified. 


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Executive summary 

Background 

Cadmium plating is the preferred protective treatment for use on aerospace components 

and fasteners manufactured from steel. Cadmium plating provides a high level of 

corrosion protection to steel being both a good barrier coating and a sacrificial coating 

and is galvanically compatible with aerospace aluminium alloys. Its low coefficient of 

friction makes it an attractive coating for use on fasteners and threaded parts. Concerns 

about the toxicity of cadmium and its harmful effects on humans and the environment in 

general has led to European legislation banning the use of cadmium plating for many 

engineering applications. Efforts are being made to identify alternatives to cadmium 

plating for aerospace applications. This report describes the results of research 

undertaken as part of a Garteur collaborative programme to evaluate a number of 

commercially available coatings. The organisations participating in the programme were 

DERA (now QinetiQ), Short Brothers and British Aerospace from the United Kingdom, 

Aerospatiale (France), SAAB-SCANIA AB (Sweden), NLR and Fokker Aircraft BV (The 

Netherlands) and Daimler Benz Aerospace Airbus (Germany). The work was carried out 

under Action Group AG17 "Cadmium Substitution" set up under the Structural Materials 

Panel of Garteur. 

Coatings evaluated 

Coatings evaluated in the programme were either aluminium or zinc based and were 

produced commercially. A reference electrodeposited cadmium coating was included in 

the programme together with an experimental aluminium - magnesium coating produced 

by unbalanced magnetron sputtering. Two aluminium coatings were selected; one 

produced by ion vapour deposition and the other prepared using a non-aqueous electrodeposition 

process. A zinc-nickel coating deposited using an acid electroplating process 

and an electrodeposited zinc-cobalt-iron coating were both evaluated in the programme. 

In addition two metallic - ceramic coatings were investigated one containing aluminium 

flakes in an oxide matrix and one consisting of aluminium and zinc particles in an 

inorganic matrix. For most of the studies conducted the coatings were applied to simple 

steel test panels or aerospace grade fasteners. 

Each of the coatings have been evaluated to determine their suitability for use on 

aerospace components and in addition to properties such as corrosion resistance, 

lubricity, galvanic compatibility and the effect of coatings on fatigue strength the 

resistance of coatings to aircraft fluids and paint adhesion have been studied. 

Optical and scanning electron microscopy techniques were used to determine coating 

thicknesses and microstructures. The results obtained indicated that the coatings had 

been applied in accordance with the procedures recommended by the various coating 

suppliers. 

Electrical conductivity measurements were made using two simple lap joints one 

employing Hi-Lok fasteners and one countersink screws. With the exception of a 

metallic-ceramic coating incorporating zinc particles, all the coatings tested gave 

electrical resistances of less than 1mΩ. The data indicate that it is possible to achieve 

good electrical conductivity with the other coatings. A low temperature tensile test 

method indicated that the adhesion of the coatings to grit blasted steel substrates was 

good. 

The corrosion resistance of each of the coatings was determined using a range of 

accelerated corrosion tests, electrochemical measurements and outdoor exposure trials. 

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The barrier properties of the coatings were determined from electrochemical 

measurements. These established that the zinc alloy coatings and zinc based metallicceramic 

coatings were similar in performance to electroplated cadmium. The aluminium 

based coatings all gave much lower corrosion currents implying that they were more 

effective barrier coatings. The sacrificial properties of the coatings were assessed from 

open circuit potential experiments and from the use of scratch model specimens and 

protection distance measurements. The main conclusions were that pure aluminium 

coatings and an aluminium based metallic-ceramic coating were less effective than 

cadmium plating. Overall the zinc alloy coatings were more effective than the aluminium 

coatings. 

Two methods were employed to study the galvanic compatibility between coatings and 

aerospace aluminium alloys. In the first coated bolts inserted into aluminium alloy blocks 

were exposed to neutral salt fog and at an outdoor exposure site. The results obtained 

indicated that the zinc based metallic-ceramic coatings were the most promising as no 

rusting was detected. The second approach used was to measure the galvanic current 

developed between coated panels and aluminium alloy. The results obtained indicate 

that ED aluminium and ED zinc-nickel coatings lower the corrosion rate of the aluminium 

alloy. Other coatings studied were found to accelerate the rate of corrosion above that 

found for cadmium plating. 

The effects of coating on fatigue performance were assessed using notched specimens 

tested under constant amplitude tests. It was established that for the ED aluminium, the 

two metallic-ceramic coatings and UBMS aluminium - magnesium coatings the reduction 

in fatigue strength was less than 5%. ED zinc-cobalt-iron and cadmium gave similar 

reductions of ~10% whilst the ED zinc-nickel coatings had the most detrimental effect 

being ~25%. 

Sustained load tests conducted on coated notch specimens, exposed to sodium chloride 

solution, indicated that the zinc based metallic-ceramic and ED zinc-cobalt -iron coatings 

may promote stress corrosion cracking in high strength steels. Additional tests carried 

out including the slow bend test suggest that any susceptibility to hydrogen 

embrittlement may be minimised by heat treatment after electroplating. 

Most of the replacement coatings examined failed to show any significant degradation on 

exposure to a range of chemicals used on aircraft. Exceptions were ED zinc-nickel 

coatings in contact with Turco 5948 and ED zinc-cobalt coatings immersed in Skydrol 

hydraulic fluid. Cadmium plating was also found to be degraded by these fluids. 

Cross-cut tests show that good paint adhesion may be achieved with metal coatings. An 

important factor is the time delay between passivation and the application of a primer. 

Data obtained indicate that if the passivated surfaces are exposed to the atmosphere for 

too long, poor paint adhesion may be obtained. 

Tribological studies were conducted to allow the suitability of the different coatings for 

use on fasteners to be established. The coefficient of friction of several of the coatings 

has determined and the torque-tension characteristics of coated fasteners after repeated 

tightening and untightening has been compared. It is concluded that only the zinc based 

metallic-ceramic coatings have a coefficient of friction comparable to cadmium plating. 

All the coatings examined resulted in pre-loads on Hi-Lok fasteners greater than the 

minimum 4kN required. With the ED cadmium, ED aluminium, ED Zn-Co-Fe and ED Zn- 

Ni coated steel fasteners the maximum preload of 10kN was exceeded. 

The use of brush plating to repair several of the coatings was investigated. Simulated 

corrosion damage and re-plating tests showed that brush plated zinc-cobalt and zinc- 


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nickel coatings could be used to re-protect a range of Garteur coatings including zincnickel, 

zinc-cobalt-iron, PVD aluminium and cadmium. 

Environmental and cost studies 

A environmental study was undertaken to establish the effect of the coatings evaluated 

on workers involved in the manufacture and maintenance of aerospace components and 

the impact of the coatings on the environment. It is concluded that none of the alternative 

coatings considered in this programme present major environmental problems or offer a 

health risk to workers involved in the application of coatings or in the assembly and 

maintenance of aerospace components. Several of the processes however involve the 

use of post plating treatments containing hexavalent chromium compounds, which could 

cause handling problems. Some tentative costs have been calculated for the 

replacement coatings and these are generally similar to the cost of cadmium plating. 

Conclusions and future work 

It is concluded that none of the coatings evaluated gave an overall performance 

equivalent to cadmium plating. Several of the coatings may have uses as substitutes for 

cadmium plating in particular applications. The major problem areas remain the 

replacement of cadmium on fasteners and threaded parts and it is recommended that 

further work should focus on these requirements. The use of multilayered coatings based 

on electrodeposited zinc alloy coatings and unbalanced magnetron sputtered aluminium 

alloy coatings is suggested. 

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List of contents 

Page 

Authorisation 

Abstract 

Executive summary 

iii 

v 

vi 

1 Introduction 1 

2 Coatings evaluated 3 

3 Summary of work programme 4 

3.1 Coating characteristics 4 

3.2 Resistance to corrosion 4 

3.3 Galvanic compatibility 4 

3.4 Effect of coatings on fatigue performance 5 

3.5 Stress corrosion cracking and hydrogen embrittlement studies 5 

3.6 Resistance to aircraft chemicals 5 

3.7 Paint adhesion 6 

3.8 Tribological properties 6 

3.9 Coating repair 6 

4 Summarised results 7 

4.1 Coating characteristics 7 

4.2 Resistance to corrosion 7 

4.3 Galvanic compatibility 11 

4.4 Effect of coatings on fatigue performance 13 

4.5 Stress corrosion cracking and hydrogen embrittlement studies 13 

4.6 Resistance to aircraft chemicals 13 

4.7 Paint adhesion 13 

4.8 Tribological properties 13 

4.9 Coating repair 14 

5 Environmental survey 15 

5.1 Effect on workers 15 

5.2 Effect on outside environment 15 

5.3 Conclusions 16 

6 Coating costs 17 

7 Discussion 18 

8 Conclusions 19 


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Page 

9 Recommendations 20 

10 References 21 

11 Tables 22 

12 Figures 35 

ANNEX A Coatings and methods of coating application 39 

A.1 Introduction 39 

A.2 Coating preparation 39 

A.3 References 41 

A.4 Tables 42 

ANNEX B Coating characteristics 47 

B.1 Introduction 47 

B.2 Test procedures 47 

B.3 Results 48 

B.4 Conclusions 50 

B.5 References 50 

B.6 Tables 51 

B.7 Figures 54 

ANNEX C Corrosion resistance 57 

C.1 Introduction 57 

C.2 Test procedures 57 

C.3 Results 59 

C.4 Conclusions 61 

C.5 References 61 

C.6 Tables 63 

C.7 Figures 70 

Page 

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ANNEX D Galvanic compatibility 71 

D.1 Introduction 71 

D.2 Test procedures 71 

D.3 Results and discussion 72 

D.4 Conclusions 73 

D.5 References 73 

D.6 Tables 74 

D.7 Figures 76 

ANNEX E Effects of coatings on fatigue performance 79 

E.1 Introduction 79 

E.2 Test procedures 79 

E.3 Results and discussion 80 

E.4 Conclusions 80 

E.5 References 81 

E.6 Tables 82 

E.7 Figures 83 

ANNEX F Stress corrosion cracking studies 85 

F.1 Introduction 85 

F.2 Test procedures 85 

F.3 Results and discussion 86 

F.4 Conclusions 87 

F.5 References 87 

F.6 Table 88 

Page 

ANNEX G Resistance to aircraft chemicals 89 


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G.1 Introduction 89 

G.2 Test procedures 89 

G.3 Results 89 

G.4 Conclusions 89 

G.5 Tables 90 

ANNEX H Paint adhesion 93 

H.1 Introduction 93 

H.2 Test procedures 93 

H.3 Results 93 

H.4 Conclusions 94 

H.5 References 94 

H.6 Tables 95 

ANNEX I Tribological properties 97 

I.1 Introduction 97 

I.2 Torque-tension measurements 97 

I.3 Reusability tests 97 

I.4 Coefficient of friction 98 

I.5 Conclusions 98 

I.6 References 98 

I.7 Tables 100 

I.8 Figures 102 

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Page 

ANNEX J Coating repair 105 

J.1 Introduction 105 

J.2 Experimental 105 

J.3 Results 105 

J.4 Conclusions 106 

J.5 Tables 107 

J.6 Figures 109 

Distribution list 111 


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1 Introduction 

Cadmium plating is the preferred protective treatment for aerospace components and 

fasteners manufactured from steel [1,2]. It offers a high general corrosion resistance, 

provides sacrificial protection if the coating is damaged and is galvanically compatible 

with aluminium alloys used in airframe structures. The high lubricity of cadmium plating 

makes it particularly attractive for fastener applications and for use on threaded parts. 

Although the electrodeposition of cadmium may result in the introduction of hydrogen 

into the steel substrate, effective heat treatments have been developed which eliminate 

the possible risk of hydrogen embrittlement in high strength steels. 

The main disadvantage of cadmium plating is the toxicity of cadmium salts. Cadmium is 

considered to impose a serious environmental and health hazard during production, 

application and use. The toxic effects of cadmium can lead to kidney deficiencies, 

damage to the lungs and cardiac system and deformity of the spinal column. 

Cyanide containing baths are normally used for the electroplating of cadmium. The 

treatment of effluent from the plating process to reduce the level of cadmium in solution 

is an expensive process, one which will become more costly as the permissible level of 

cadmium is decreased. Compared with other industries the aerospace industry is in an 

exceptional position. The use of cadmium plating for general engineering purposes is no 

longer allowed under European Legislation but it may still be used for aerospace and 

military applications where there is no acceptable alternative [3]. Member states of the 

European Union are seeking to widen the ban on the use of cadmium and it is likely that 

future directives may be extended to include the aerospace industry. It was against this 

background that an exploratory Garteur group was formed in 1993 to consider the setting 

up of a collaborative programme to evaluate alternatives to cadmium plating for use on 

aerospace components. A programme of work was agreed between the interested 

parties and an action group AG17 “Cadmium Substitution” was formally set up under the 

Structural Materials Panel of Garteur. Research commenced on 1st January 1994 and 

most of the experimental work was completed by the 31st December 1996. 

Initially eight organisations from five countries participated in the programme. These 

were 

• Aérospatiale (France) 

• British Aerospace, Military Aircraft Division (United Kingdom) 

• Daimler Benz Aerospace Airbus (Germany) 

• DERA (United Kingdom) 

• Fokker Aircraft BV (The Netherlands) 

• NLR (The Netherlands) 

• SAAB -SCANIA AB (Sweden) 

• Short Brothers PLC (United Kingdom) 

During the second half of the programme Fokker Aircraft BV ceased trading and were 

forced to withdraw from the programme. Outstanding experimental work was completed 

by NLR. 

The programme has been primarily concerned with the testing and evaluation of selected 

coatings. The main objective was to identify alternatives to cadmium plating, which may 

be applied to steel components and fasteners, used in aircraft structures. 

GARTEUR SM/AG17 TP128 Page 1


The various aims of the programme were as follows:- 

• to select a range of coatings 

• to agree on the test specifications to be used 

• to assess the performance of each coating 

• to establish the technical, environmental and economic aspects of each 

coating 

• to complete the programme within a three year time-scale 

A paper was presented on the progress of the research at the AGARD meeting on 

"Environmentally Compliant Surface Treatments of Materials for Aerospace Applications" 

in 1996 [4]. This report presents the results of the experimental programme and gives 

assessments of the environmental and economic aspects of the various coatings 

evaluated. Summarised results are given in the main text of the report and more detailed 

information about test procedures and test data are included in a series of annexes. 

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2 Coatings evaluated 

An important feature of cadmium plating is its ability to provide sacrificial protection if it 

becomes damaged in service. In considering the range of coatings that might be 

substituted for cadmium only coatings that would provide sacrificial protection were 

selected. Thus nickel plating, which can provide a high level of corrosion protection, was 

not chosen as the coating is more noble than steel and gives no protection once the 

coating is damaged. Details of the coatings evaluated together with the method of 

application are given in table 1. 

The coatings are either aluminium or zinc based and with the exception of the 

unbalanced magnetron sputtered aluminium - magnesium coating are available 

commercially. Electro-deposited cadmium plating has been included in the programme 

as a reference. 

The coatings were applied to 150mm x 100mm panels cut from 1mm thick 4130 steel 

sheet. These panels were used to evaluate the corrosion resistance of the coatings and 

properties such as paint adhesion and resistance to aircraft fluids. A series of fasteners 

were also coated to allow torque-tension and electrical conductivity measurements to be 

made. Fatigue specimens and hydrogen embrittlement test pieces were machined from 

4340 steel rods and subsequently plated. 

Detailed information regarding the application of the various coatings is given in Annex 

A. 



3 Summary of work programme 

Each of the coatings listed in table 1 has been evaluated to determine their suitability for 

use on aerospace components. Hence in addition to properties such as corrosion 

resistance, lubricity, galvanic compatibility etc. the effect of coatings on fatigue strength, 

resistance to aircraft fluids and the risk of hydrogen embrittlement resulting from coating 

process have been investigated. The various aspects of coating performance, which 

have been studied, are summarised in table 2. Annexes B to J give detailed descriptions 

of the various tests employed and the results obtained. 

In the first phase of the programme coatings were applied to 150mm x 100mm panels 

cut from 1mm thick 4130 steel sheet. These panels were used to evaluate the corrosion 

resistance of the coatings and properties such as paint adhesion and resistance to 

aircraft fluids. A series of fasteners were also coated to allow torque-tension and 

electrical conductivity measurements to be made. Fatigue specimens and hydrogen 

embrittlement test pieces were machined from 4340 steel rods and subsequently plated. 

3.1 Coating characteristics 

Optical and scanning electron microscopy techniques were employed to determine the 

microstructure and surface roughness of the coatings. The adhesion and flexibility of 

coatings to the substrate were measured using cross-cut tests. 

Electrical conductivity measurements of bolted joints were made using two specimen 

configurations, details of which are given in Annex B. Joints formed between two painted 

aluminium alloy strips using both coated Hi-Lok and countersink fasteners were studied. 

In each case two measurements, type A and type B were made. For the type A 

measurements, the electrical resistance between the ends of the two painted strips was 

determined, whilst for the type B measurements the electrical resistance between the 

end of one painted strip and the fastener was measured. 

3.2 Resistance to corrosion 

Cadmium plating acts both as barrier coating effectively isolating the steel substrate from 

the environment and as a sacrificial coating. This ensures that if the coating is damaged 

the exposed substrate is cathodically protected. These two aspects of coating 

performance have been evaluated 

A range of accelerated and outdoor exposure tests and electrochemical measurements 

have been used to compare the corrosion resistance of the various coatings with 

electrodeposited cadmium plating. More detailed information about the corrosion test 

programme are given in Annex C. 

3.3 Galvanic compatibility 

Two approaches have been employed to investigate the compatibility of replacement 

coatings with structural aluminium alloys. In the first, neutral salt spray tests and outdoor 

exposure trials have been conducted on samples consisting of aluminium blocks into 

which have been inserted coated fasteners. The second approach has been to measure 

directly the corrosion current developed between selected coatings and 2000 and 7000 

series aluminium alloys. Annex D describes in more detail the galvanic corrosion 

measurements. 

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3.4 Effect of coatings on fatigue performance 

Surface treatments such as pickling, anodising and plating may have a detrimental effect 

on the fatigue life of aerospace components. Constant amplitude fatigue tests have 

been made to determine the effects of the different coatings on the fatigue life of 

specimens machined from AISI 4340 steel tempered to give a tensile strength of 1400 

MPa. Both smooth specimens and notched specimens were used to give K t (stress 

concentration) values of 1.0, 1.4, 2.5 and 4.0. Tests were conducted at a frequency of 

185Hz and a stress ratio equal to 0.1. Details of the fatigue programme are given at 

Annex E. 

3.5 Stress corrosion cracking and hydrogen embrittlement studies 

High strength steels are susceptible to hydrogen embrittlement. Electroplating 

processes such as cadmium plating are less than 100% efficient and some hydrogen will 

be evolved during deposition, which may diffuse into the steel substrate. To reduce the 

risk of hydrogen embrittlement a post plating heat treatment is carried out on all steels 

with a strength greater than 1400 MPa. In the case of a steel part manufactured from a 

steel with a strength of 1850 MPa and electroplated with cadmium this would involve 

baking at 200 o C for a minimum of 18 hours. In the current programme, tests are being 

conducted on coated steel specimens to identify potential hydrogen embrittlement 

problems. After coating the samples are de-embrittled in accordance with the plating 

specification or the coating suppliers instructions. 

Two aspects of hydrogen embrittlement have been investigated. In the case of coatings 

prepared by electrodeposition, there is concern that the process itself may generate 

sufficient hydrogen to cause cracking under tensile loading. To investigate this sustained 

load testing and slow bend tests were carried out on coated samples. A second 

consideration is the possible introduction of hydrogen into the steel substrate as a result 

of corrosion occurring on the coating. This has been studied using notched tensile 

specimens. These are loaded to 75% of the notched tensile strength after coating and 

are then subjected to alternate immersion in 3.5% sodium chloride solution until failure 

occurs. The performance of each of the coatings is being compared with cadmium 

plating. Annex F describes the two test procedures employed and the results obtained. 

3.6 Resistance to aircraft chemicals 

Hydraulic fluids, aviation fuel, paint strippers and many of the other chemicals and liquids 

used on aircraft and in the maintenance of aircraft are potential corrosion hazards to the 

airframe structure. Immersion tests were carried out to establish the degree to which 

coatings degrade when exposed to some of the more commonly used aircraft chemicals. 

The range of chemicals includes 

• ethylene glycol 

• aviation fuel 

• butyl phosphate type hydraulic fluid 

• alkaline based general purpose cleaner 

Two test procedures have been employed both based on the immersion of coated 

panels in different aircraft fluids. The detailed test procedures are given in Annex G 

together with the test results. 

3.7 Paint adhesion 



The adhesion of conventional epoxy based primers to passivated coatings was 

examined using a standard cross hatch test. In each case the primer was applied to the 

complete coating scheme. For example in the case of the electrodeposited zinc – nickel 

and zinc – cobalt –iron coatings these were passivated prior to painting. Further details 

of the test procedures employed are given in Annex H. 

3.8 Tribological properties 

The low coefficient of friction of cadmium plating makes it an ideal coating for use on 

fasteners and threaded parts. Much of the torque used in tightening up a bolt is 

expended in overcoming the frictional constraints. Coatings with a high lubricity are 

therefore necessary if adequate pre-loads are to be achieved without the application of 

excessive torque levels. Another advantage obtained with cadmium plated fasteners is 

reproducible torque-tension characteristics. During the assembly of an aircraft, parts are 

often bolted into place and then removed for adjustment or modification. This process 

may be repeated a number of times and it is essential that there is no significant change 

in the torque-tension behaviour of the fasteners employed. In the current programme the 

effect of repeated tightening and untightening on the torque – tension behaviour of 

coated steel bolts and nuts was determined. Details of the tests are given in Annex I. 

3.9 Coating repair 

Trials have been undertaken to examine the repair of coatings damaged in service. 

Brush plating techniques, for instance, were evaluated as a method of re-plating areas 

on zinc – nickel electroplated parts. In preliminary work bath plated test panels have 

been exposed to neutral salt fog and any corroded areas, after cleaning, re-protected by 

brush plating. The panels were then further exposed to a salt fog environment. Repair 

methods for other coating systems were also be evaluated (See Annex J). 

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4 Summarised results 

4.1 Coating characteristics 

Three aspects were evaluated 

• Microstructure and composition 

• Electrical conductivity 

• Coating adhesion 

4.1.1 Microstructure and composition 

The microstructures of the commercially produced coatings were consistent with those 

described by the various supply houses and reported in the open literature. Table 3 

briefly summarises the general appearance of the microstructures and gives details of 

the coating thicknesses and compositions. The thickness measurements refer to the 

unpassivated coatings and give the range of values determined by the partners. 

4.1.2 Electrical conductivity 

Results of the electrical conductivity measurements are summarised in table 4. 

Electrical resistance values were obtained for type B measurements made on joints 

made using both Hi-Lok fasteners and countersink screws. With the exception of the 

Delta-tone coatings, values of electrical resistance of less than 1mΩ were obtained. For 

the type A measurements resistance values could only be determined for ED cadmium, 

ED zinc-nickel and ED zinc-cobalt-iron coated Hi-Lok fasteners and ED zinc-nickel 

coated countersink screws. 

No measurements were made on either the ED aluminium or UMS aluminium – 

magnesium coatings but it is likely that their performance will be similar to PVD 

aluminium. The SermeTel 984 coating was not included in the electrical conductivity test 

as it is not intended as a coating for fastener applications. 

4.1.3 Coating adhesion 

Tests conducted by NLR on ED cadmium, ED zinc-cobalt-iron, ED zinc-nickel and 

SermeTel 984 coated test specimens indicated that adhesion to grit blasted steel 

substrates was very good. 

4.2 Resistance to corrosion 

The detailed corrosion test results are given in Annex C. Two aspects of coating 

performance have been assessed:- 

• Coating barrier properties 

• Sacrificial properties 

4.2.1 Electrochemical measurements 

The true barrier properties of the coatings may only be determined using electrochemical 

methods, which allow the corrosion rate of a coating in contact with a corrosive 

environment to be measured. In accelerated corrosion and outdoor exposure trials the 

time to first rust on undamaged coated panels is dependant on both the barrier 



properties and sacrificial properties of the coating. The thickness of the coating will also 

have a major impact on the performance. 

The results in table 5 indicate that the corrosion resistance of the electro-deposited zinc 

alloy coatings, in both the as plated and passivated conditions, are similar to that of 

electroplated cadmium. The Delta-tone coating, which contains zinc flakes, gives a 

corrosion current similar to the two zinc alloy coatings. The aluminium based coatings all 

gave much lower corrosion currents indicating that they are more effective barrier 

coatings. Corrosion current data obtained by NLR and BAe for different chloride 

concentrations gave similar results. 

An indication of the sacrificial properties of the coatings can be obtained from the 

measurement of the open circuit potential in sodium chloride solution. Tests were made 

by DBAA, DERA, NLR and BAe in various concentrations of sodium chloride solution. 

Detailed results are given in table C11 in Annex C. Figure 1 shows the differences in 

open circuit potential compared to cadmium plating. 

The data in figure 1 indicate that the zinc rich coatings i.e. ED zinc-nickel, ED zinccobalt-iron 

and Delta-tone have lower open circuit potentials that ED cadmium and would 

be expected to provide a greater degree of sacrificial protection than cadmium. 

Two sets of experiments were conducted at DERA in order to quantify the sacrificial 

properties of the coatings. The first approach was to use a scratch model test piece. This 

allows the galvanic current developed between the coating and the steel substrate to be 

monitored. Details of the specimen design and the technique are given in Appendix C. 

The second approach used was to measure the distance over which the coating is able 

to provide protection to the exposed substrate. Again details of the technique are given 

in Appendix C. 

Figures 2 and 3 compare the galvanic currents and the protection distances for both the 

unpassivated and passivated coatings respectively. 

The results presented in figures 2 and 3 indicate that the two pure aluminium coatings 

together with the SermeTel coating are less effective as sacrificial coatings than 

cadmium plating. Some improvement is achieved through the addition of magnesium. 

The results are generally in line with the behaviour predicted from the open circuit 

corrosion potential measurements given in figure 1. A low value for the open circuit 

potential of the ED aluminium was recorded in the present study. However previous 

electrochemical studies on pure aluminium have shown that the potential may fall rapidly 

with the initiation of corrosion pits [1]. Overall the electrodeposited zinc alloy coatings 

and the zinc containing Delta-tone coating give a greater level of sacrificial protection 

than the pure aluminium coatings. Values for the open circuit potential shown in figure 

4.1 for these coatings are much lower than for cadmium plating indicating that they 

should provide a high degree of sacrificial protection. 

4.2.2 Corrosion tests on undamaged panels 

Accelerated corrosion tests and outdoor exposure trials were made on both undamaged 

and scratched coated test panels. The time to first rust was used to compare the 

performance of the different coating systems. In the case of the undamaged panels this 

will depend both on the barrier and the sacrificial properties of the coatings. Their relative 

importance will depend on the nature of test, for example whether the coating is 

continuously exposed to a corrosive environment or whether there is a drying cycle. 

With the scratched panels, the occurrence of rusting in the scratch is used to assess the 

sacrificial properties of the coating. 

A) Accelerated corrosion tests 

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The results of the neutral salt fog tests undertaken by DERA are given in figure 4. The 

data show that only the two commercial metallic ceramic coatings, SermeTel and Deltatone 

out performed cadmium when tested in the as plated condition. In every case 

carrying out post plating treatments such as passivating considerably improved the 

performance of the coating but only the SermeTel 984 coating with SermeTel 985 gave a 

higher time to first red rust than passivated cadmium plating. 

Neutral salt spray tests were conducted by two other laboratories Shorts and Fokker on 

a limited number of coatings. Although the actual times to red rust for undamaged panels 

varied considerably between the three laboratories, there was general agreement 

regarding the relative performances of the coatings. 

As plated 

SermeTel 984 > ED cadmium >ED zinc – nickel ˜ ED zinc – cobalt – iron 

Plated + post plating treatment 

SermeTel 984 + 985 > Pass. ED cadmium > Pass. ED Zn-Ni ˜ Pass. ED Zn-Co-Fe 

As indicated in section 4.1, the thicknesses of the different coatings studied varied 

considerably. The thicknesses were specified by the various coating suppliers to give the 

desired combinations of corrosion protection, adhesion, frictional properties etc. The 

SermeTel 984 coating for example has a mean thickness of about 40μm compared with 

PVD aluminium, which was found to be 18 – 22μm thick. Previously published work has 

looked at the dependence of time to red rust on coating thickness [2]. This shows that for 

relatively thin coatings an almost linear relationship exits. However for thicker coatings, 

small increases in coating thickness have a large effect on the time to red rust. 

In the present study, the results given in figure 4 have been compared taking into 

account the differences in coating thicknesses. A “normalised” time to red rust was 

calculated for each coating system by dividing the time to red rust by the coating 

thickness. The values obtained are compared in figure 5. 

From figure 5 it is apparent that whilst the as plated UMS Al-Mg, Delta-tone and 

SermeTel coatings out perform cadmium plating in the neutral salt fog tests, the 

passivated cadmium plating gives the highest level of corrosion protection. 

In the present programme two additional accelerated corrosion tests were employed to 

evaluate the coatings. These were 

• Exposure to intermittent acidified salt spray (MASTMAASIS test) 

• Exposure to 100% humidity 

MASTMAASIS testing was undertaken by DERA and NLR whilst exposure to 100% 

humidity was investigated by Fokker. 

The main observations from the MASTMAASIS tests are summarised in table 6. Whilst 

good agreement was obtained for tests conducted on as plated panels some differences 

were found on tests carried out on passivated panels. 

Humidity tests were conducted on the ED zinc alloy coatings and on the two metal – 

ceramic coatings. Testing was continued for 2000 hours and at this point only the Deltatone 

showed evidence of rusting. 

B) Outdoor exposure trials 



Outdoor exposure trials were conducted at two sites, Schiphol in the Netherlands and at 

Portsmouth in the UK. The Schiphol site may be described as mildly industrial whilst the 

Portsmouth site is located approximately 50 metres from the sea. 

Exposure trials at Schiphol were limited to the ED zinc alloy coatings and the two metalceramic 

coatings. At the end of the 462 day exposure period red rust was only found on 

the as plated ED zinc-nickel coated panels. Trials conducted at Portsmouth covered all 

the coatings being studied in the programme and continued for approximately two years. 

At the end of this period the performance the following coatings showed no evidence of 

red rust:- 

• ED cadmium (as plated) 

• ED cadmium (plated + passivation) 

• ED zinc – nickel (plated + passivation) 

• ED zinc – cobalt – iron (as plated) 

• ED zinc – cobalt – iron (plated + passivation) 

• Delta-tone 

• SermeTel 984 

• SermeTel 984/985 

The PVD and ED aluminium coatings and UMS aluminium – magnesium coatings 

generally performed poorly in comparison with electroplated cadmium. 

4.2.3 Corrosion tests on scribed panels 

Both accelerated corrosion tests and outdoor exposure trials were conducted on coated 

panels scribed to expose the steel substrate. The time to first rust in the scribe region 

was used as a measure of the ability of the coating to provide sacrificial protection. 

A) Accelerated corrosion tests 

Results obtained from neutral salt fog testing showed large variations in time to rust 

between the three laboratories carrying out the tests. For the purposes of this 

investigation, the salt fog test has been used simply to rank the various coatings in terms 

of their ability to sacrificially protect the substrate. The results obtained are summarised 

in table 7. The main area of difference between the test results concerns the relative 

performances of the SermeTel 984 and SermeTel 984/985 coatings. In the DERA tests 

rusting in the scribe was detected after 168 hours exposure whilst in the Shorts and 

Fokker trials times exceeding 2000 hours were reported (see Annex C). Electrochemical 

measurements indicate that the potential difference between the coating and substrate is 

much lower than for the zinc based coatings. Relatively short times to rusting might 

therefore be expected. The discrepancy between the results may possibly be associated 

with differences in the width of the scribe. This will be more significant with thick coatings 

than with thinner coatings such as ED zinc-nickel. 

With the exception of the SermeTel coatings, the general pattern of results is consistent 

with the findings of the electrochemical studies. These established that the zinc based 

systems provide a higher level of sacrificial protection than pure aluminium coatings. 

B) Outdoor exposure trials 

Coated panels were exposed at three outdoor test sites as discussed in section 4.2.2 

above. Table 8 compares the relative performance of each coating. The results obtained 

Page 10 



show that in each case the ED zinc-cobalt-iron coatings gave the greatest degree of 

protection in the scribed region overall equalling the performance of ED cadmium. The 

ED zinc-nickel was not so effective. As indicated in Annex C, rusting was found in the 

scribe areas of all the zinc-nickel coated panels tested at the three outdoor exposure 

sites. Delta-tone coatings were exposed at two of the sites and were found to compare 

favourably with ED cadmium plating. The pure aluminium coatings and the SermeTel 

coatings were the least effective and gave a level of protection well below that found for 

cadmium plating. 

The results obtained under outdoor exposure conditions are consistent with the electrochemical 

measurements reported in section 4.2.1 above which showed that the zinc 

based coatings were more effective as sacrificial coatings than the aluminium based 

systems. 

4.2.4 Conclusions 

1. The aluminium based coatings were found to have superior barrier properties to the 

zinc rich deposits 

2. Electrochemical and accelerated corrosion tests have demonstrated that the zinc 

rich deposits are more effective in preventing the onset of rusting at scratches or defects 

in the coating than the aluminium and aluminium alloy coatings. 

4.3 Galvanic compatibility 

Two methods have been employed to study the galvanic compatibility between the 

coatings and aerospace aluminium alloys. In the first coated bolts have been inserted 

into aluminium alloy blocks and exposed to a corrosive environment. Research at DBAA 

used exposure to neutral salt fog whilst at Fokker test blocks were also placed on test at 

the Schiphol outdoor exposure site. The second method employed directly measured 

the galvanic current generated when a coated sample was connected to an aluminium 

alloy panel. 

4.3.1 Corrosion tests on bolt/block specimens 

Results of the neutral salt fog tests on bolt/block specimens indicated that several 

coatings caused less dissimilar metal corrosion than cadmium plating. The results of the 

salt spray tests based on the data given in Annex D are summarised in table 9. 

4.3.2 Electrochemical studies 

Galvanic compatibility tests were carried out on coated panels electrically connected to 

aluminium alloy coupons and immersed in 600mmol/litre sodium chloride solution for 168 

hours. The galvanic corrosion current generated was monitored throughout the test and 

the mean corrosion current calculated. The effect of different coatings on the corrosion 

rate of the aluminium alloys was also determined by measuring the weightloss of the 

aluminium alloy coupons used in the galvanic corrosion tests. The values obtained were 

compared with weightloss measurements made on uncoupled test coupons. 

4.3.3 Galvanic corrosion current measurements 

From the galvanic corrosion current measurements, two types of behaviour were 

observed. For the as plated zinc based coatings (ED zinc-nickel, ED zinc-cobalt-iron and 

Delta-tone) the galvanic corrosion current recorded initially was high but fell during the 

168 hour monitoring period. In the case of the ED zinc-nickel coating, current reversal 

occurred and is associated with de-zincification of the coating. The passivated zinc alloy 



coatings showed an initial drop in current during the first 50 hours or so of immersion but 

after this period the current appeared to stabilise. 

For the cadmium plated and aluminium based coatings including SermeTel the pattern of 

behaviour was different. The galvanic corrosion current remained fairly stable throughout 

the monitoring period. 

4.3.4 Weightloss measurements 

The weightlosses determined at the end of the 168 hour immersion period were used to 

compare the effects of the coatings on the corrosion rate of the two aluminium alloys. 

Table 10 identifies those coatings that increased the corrosion rate compared with 

cadmium plating, coatings which were similar to cadmium plating and coatings which 

reduced the corrosion rate of the aluminium alloy. 

The results presented in table 10 suggest that ED aluminium and ED zinc-nickel are 

overall similar in performance to ED cadmium plating and give galvanic compatibility with 

aluminium alloys. The other coatings examined in the as plated condition tend to 

increase the corrosion rate of the aluminium alloy. 

4.3.5 Discussion 

For many aerospace applications parts and fasteners manufactured from steel are used 

in contact with components manufactured from aluminium alloys. The role of the coating 

applied to steel parts in these situations is two fold. 

• To provide corrosion protection to the steel part 

• To minimise the galvanic corrosion current developed between the steel part 

and the aluminium alloy 

One important consideration for fastener applications is that coating should not 

accelerate the corrosion of the substrate into which it is inserted. Clearly the risk of 

dissimilar metal or galvanic corrosion may be minimised by selecting coatings which 

have open circuit potentials close to that of the 2000 and 7000 series aluminium alloys 

currently employed on aircraft. On this basis, ED cadmium plating and coatings based on 

aluminium should give the lowest risk of galvanic corrosion. This is supported by the 

galvanic corrosion measurements, which show that small but relatively constant currents 

are generated when coated coupons are connected to aluminium alloys immersed in 

chloride solutions. Much higher galvanic corrosion currents are generated when zinc 

based coatings are tested. This reflects the much greater differences in potential which 

exist between these coatings and aluminium alloys. One aspect of the performance of 

ED zinc alloy coatings which needs to be considered is the loss of zinc or de-zincification 

which takes place during corrosion. This has the effect in the case of ED zinc-nickel 

coatings of raising the nickel content of the alloy and hence the corrosion potential. The 

difference in electropotential between the coating and aluminium alloy is lowered 

resulting in a reduction in galvanic corrosion current. 

Table 10 indicates that a number of the coatings examined accelerated the corrosion of 

the aluminium alloys despite being more active than aluminium. This is due to the build 

up of alkaline corrosion conditions at the surface of the aluminium alloy electrodes 

resulting in pitting attack. From the data in appendix D it would appear that the level of 

alkaline attack is related to the magnitude of the galvanic corrosion current. The ED zinccobalt-iron 

coating, for example, which generated a high initial galvanic corrosion current 

accelerated the corrosion of the aluminium alloy electrodes. 

Page 12 



The results of the bolt/block trials indicate that the Delta-tone coating is the most 

promising since no rusting was detected. Rusting was observed on the other four 

coatings examined when no passivation treatment was applied. 

4.4 Effect of coatings on fatigue performance 

The results of the constant amplitude fatigue tests conducted by SAAB, British 

Aerospace and NLR are given in detail in Annex E. 

The general effects of the coatings on fatigue strength are summarised in table 11. This 

indicates that the greatest reduction in fatigue strength occurred with the 

electrodeposited zinc-nickel coatings. Coatings such as SermeTel and Delta-tone were 

found to be less damaging than electrodeposited cadmium. 

4.5 Stress corrosion cracking and hydrogen embrittlement studies 

Results of the stress corrosion cracking tests undertaken by SAAB are given in Annex F. 

Failures were obtained for specimens protected with ED zinc-cobalt-iron and Delta-tone. 

No evidence of stress corrosion cracking was obtained with the remaining coatings. 

4.6 Resistance to aircraft chemicals 

Tests to assess the resistance of the coatings to aircraft chemicals such as hydraulic 

fluids, aviation fuels and aircraft cleaners were conducted by British Aerospace and 

Shorts. The main findings of these tests are summarised in table 12. The results show 

that several of the coatings including electrodeposited cadmium were susceptible to 

corrosion attack by the aircraft cleaner Turco 5948. Skydrol also had a detrimental effect 

on the electrodeposited zinc-cobalt-iron and cadmium coatings. 

4.7 Paint adhesion 

The results of the paint adhesion tests are presented in Annex H. The general 

conclusion was that good paint adhesion can be achieved with metal coatings. An 

important factor is the delay time between passivation and the application of a primer. 

Data obtained by NLR indicate that if the passivated surfaces are exposed to the 

atmosphere for too long, poor paint adhesion may be obtained. 

4.8 Tribological properties 

Studies of the tribological properties of the coatings concentrated on two aspects 

• torque - tension measurements on coated nuts and fasteners 

• coefficient of friction 

Detailed results are presented in Annex I. 

In the first series of torque-tension measurements the maximum preload, which could be 

obtained was recorded together with the locking torque applied. Two sets of tests were 

conducted the first using coated steel pins with coated steel collars and the second 

involving coated titanium pins with coated steel collars. In each case it was possible to 

obtain a preload in excess of 4kN which falls within the normal requirement of a preload 

in the range 4 to 9kN. 

In the second series of torque-tension measurements, tension loads developed for 

coated bolts installed with lubricant were determined. Values were obtained for locking 



torques of 7 and 17.5Nm. In each test series the load developed was highest for the 

cadmium plated bolts and nuts. However in all cases it was possible to exceed a preload 

value of 4kN. 

Limited data were obtained for the coefficient of friction for each of the coatings. A 

search of the literature yielded some further data. With the exception of the Delta-tone 

coating the coefficients of friction were found to be significantly higher than cadmium 

plating. 

4.9 Coating repair 

The use of brush plating for the repair of electrodeposited zinc-nickel, zinc-cobalt-iron 

and PVD aluminium coatings was investigated. Details of the programme are given in 

Annex J. The approach adopted was to examine the corrosion resistance of coated 

panels damaged by removing a portion of the coating at the centre of the test panel and 

repaired using either brush plated zinc-nickel or zinc-cobalt coatings. 

The tests showed that brush plated zinc-cobalt and zinc-nickel coatings could be used to 

protect a range of Garteur coatings including zinc-nickel, zinc-cobalt-iron, IVD aluminium 

and cadmium itself. Although galvanic interactions were observed for some re-plated 

coatings, these effects were minimised by application of a zincate chemical treatment. 

Page 14 



5 Environmental survey 

Surveys were made to establish the effect of the coatings being evaluated on workers 

involved in the manufacture and maintenance of aerospace components and the impact 

of the coatings on the environment. 

5.1 Effect on workers 

The survey considered the potential risk to workers from the handling of aerospace 

coated parts during the assembly and maintenance of aircraft. It also examined the risks 

to workers in metal finishing shops with regard to the application of the coatings. Tables 

13 and 14 summarise the findings of the survey. 

5.2 Effect on outside environment 

A survey of each of the coatings was made to establish the effects of the coatings on the 

environmental. Four aspects were considered:- 

• costs of disposal of waste materials 

• remnants of corroded coatings 

• waste water flow 

• air pollution 

Results of the survey are summarised in tables 15 to 18. 

Table 15 indicates that the cost of disposing of waste materials produced by the 

alternative coating processes are likely to be much lower than that incurred with 

cadmium electroplating. In the case of the two physical vapour deposition processes, 

IVD aluminium and UBMS aluminium - magnesium, accurate costs are not available. 

Several of the processes involve the use of post plating passivation treatments 

containing chromates and there will be some costs concerned with the treatment of rinse 

waters etc. 

One problem encountered with cadmium plated parts is the handling of components 

which are corroded. Cadmium corrosion products are highly toxic and may be taken into 

the body if suitable health and safety precautions are not taken. The assessment given 

in table 16 suggests that the health risk from corrosion products on the alternative 

coatings is minimal. 

Potential problems associated with the treatment and disposal of waste water from the 

various coating processes are summarised in table 17. The main conclusion is that the 

alternatives do not present major problems. Waste water from cadmium electroplating 

facilities requires considerable treatment to reduce the cadmium ion concentration to a 

level that meets environmental controls. Some problems may be encountered with the 

other coatings, which employ chromate based passivation treatments to improve 

corrosion resistance and paint adhesion. This includes the electrodeposited zinc alloy 

coatings and the PVD and electrodeposited aluminium coatings. Rinse solutions 

containing hexavalent chromium ions will need to be treated before they may be 

discharged into rivers etc. 

The possible air pollution risks arising from chemicals employed in the various coating 

processes are outlined in table 5.6. Some slight hazards result from the use of butanol in 

the production of Magni-coat and the use of toluene in the aluminium electrodeposition 

process. The risk from cadmium compounds used in plating are well documented if 

allowed to become airborne. 



5.3 Conclusions 

The surveys conducted indicate that none of the alternative coatings considered in this 

programme present major environmental problems or offer a health risk to workers 

involved in the application of coatings or in the assembly and maintenance of aerospace 

components. Several of the processes however involve the use of post plating 

treatments containing hexavalent chromium compounds, which could cause handling 

problems. 

Page 16 



6 Coating costs 

Attempts were made to compare the cost of the alternative coatings being evaluated with 

those associated with electroplated cadmium. The coating suppliers identified in table A1 

(Annex A) were asked to provide cost estimates for coating 1000 sheets 100 x 150mm. 

This information was used to calculate the cost in dollars to coat 1 square metre. 

An overview of the relative costs for commercially available coating processes is given 

below in table 19. Comparisons are made in terms of the cost in US $ per square 

decimetre. With the exceptions of the IVD aluminium and SermeTel CR984/985 coatings, 

the replacements studied are generally less costly than cadmium plating. 

Coating Cost $/dm 2 Supplier 

ED cadmium 1.59 SW Metal Finishing 

ED Zinc-nickel 1.59 SW Metal Finishing 

ED Zinc-cobalt 

SAAB 

ED Aluminium 1.13 Alcotech 

IVD Aluminium 1.94 Ion Deposition 

Delta-tone (rack) 1.25 Ewald Dörken AG 

Delta-tone (barrel) 0.25 - 0.41 Ewald Dörken AG 

SermeTel CR984/985 2.01 Sermatech 

Zinc plating ~0.12 Automotive industry 

Table 19 ; Relative costs for commercially available coatings 

For comparison purposes the cost of electroplated zinc has been included. This is used 

in many sectors including the automotive industry. The relative cost is considerably lower 

than for the two zinc alloy coatings studied, which are finding increased applications on 

car and vehicle bodies. 



7 Discussion 

In the framework of Garteur co-operation, an extended program was performed on a 

selection of coatings that could be substitutes for cadmium plating on steel. The coatings 

considered were ED zinc-nickel, ED zinc-cobalt-iron, ED aluminium, SermeTel 

CR984/985, Delta-tone/Delta-seal and ED cadmium as a reference. In addition a limited 

amount of work was performed on experimental aluminium - magnesium coatings 

applied by magnetron sputtering and on IVD aluminium. 

The aspects of the investigation were:- 

• Coating characteristics 

• Corrosion, galvanic compatibility, hydrogen embrittlement 

• Resistance to aircraft chemicals 

• Effects of coatings on fatigue 

• Tribological properties and coating repair 

• Environmental survey and relative coating costs 

The matrix of tests conducted has shown that none of the coatings evaluated give an 

overall performance equivalent to cadmium plating. Several of the coatings out perform 

cadmium plating under certain test conditions but fail to match cadmium plating in other 

tests. For example electrochemical measurements would indicate that PVD and 

electrodeposited aluminium coatings are more effective barrier coatings than cadmium 

plating. However outdoor exposure trials have established that in a marine environment 

the pure aluminium coatings give much shorter times to the onset of red rust. This is a 

consequence of the inferior sacrificial properties of pure aluminium coatings. 

Table 20 summarises the main findings of the study in terms of performance relative to 

cadmium plating. It is clear from the results obtained in the current programme that there 

is no single replacement for cadmium plating but for some applications alternatives may 

be available. For example table 20 would indicate that an electrodeposited zinc-nickel 

coating might be applied to a non-threaded part where fatigue performance was not an 

issue. 

Page 18 



8 Conclusions 

The main conclusions from the experimental investigation were: 

1. None of the coatings evaluated gave an overall performance equivalent to cadmium 

plating. 

2. Outdoor exposure tests show that ED zinc-cobalt -iron appears very similar to 

cadmium in protecting the scribed area in both marine and relatively rural 

environments. All other coatings were less effective whilst SermeTel was the least 

efficient. 

3. For a maximum resistance to corrosion attack a passivation treatment with chromium 

VI containing solution is the most promising. 

4. Galvanic compatibility with coated fasteners into aluminium alloy blocks indicate that 

the Delta-tone coating is the most promising. 

5. Resistance measurements indicate that good electrical conductivity with both Hi-Lock 

and coated countersink screws can be obtained. 

6. With the exception of the Delta-tone coating the coefficients of friction were found to 

be significantly higher than cadmium plating. 

7. The ED zinc-nickel coating caused the largest reduction in fatigue strength (25%). 

ED zinc-cobalt-iron gave no larger reduction than cadmium plating (~10%). 

8. Brush plating with zinc-cobalt and zinc-nickel electrolytes could be used for repair 

(protection) of a range of Garteur coatings. 



9 Recommendations 

The work described in this report has largely concentrated on the evaluation of 

commercially available coatings. Whilst several of the coatings may be used as 

alternatives to cadmium plating for some applications, there is no acceptable substitute 

for cadmium on fasteners and threaded parts. The main problem lies with achieving a 

combination of corrosion protection, galvanic compatibility and frictional properties from a 

single coating system. None of the coatings examined however were found to provide all 

of these properties. 

One approach now being considered is to use a dual layered coating system whereby 

the outer layer provides the necessary tribological properties and the inner layer gives 

the appropriate corrosion protection. Recent research [4] suggests that multilayered zinc 

alloy coatings can be electrodeposited by varying the current density at which the plating 

is carried out. An alternative approach might be the incorporation of a solid lubricant into 

the coating during electroplating. 

Significant advances have been made in the physical vapour deposition of metal 

coatings. The feasibility of depositing aluminium alloy coatings has been demonstrated 

using an unbalanced magnetron sputtering (UBMS) process [5]. In the present work an 

experimental aluminium - magnesium alloy coating was examined. Since the completion 

of the Garteur programme, further data have been published on the corrosion behaviour 

of aluminium - magnesium coatings in both accelerated tests and in long term marine 

exposure trials [6]. It has been shown that additions of magnesium of up to 20 weight 

percentage considerably improve the corrosion resistance of aluminium coatings. The 

UBMS deposition process can be used to produce a range of alloy coatings and may be 

adapted to give layered deposits. 

It is recommended that future Garteur collaborative work should focus on the 

development and evaluation of novel coatings for fastener applications. The research 

should examine the use of both multilayered electrodeposited zinc rich coatings and 

aluminium based coatings prepared by physical vapour deposition. In addition a survey 

should be conducted to identify any new coatings which have become available 

commercially. The programme should include a comprehensive study of the corrosion 

resistance, galvanic compatibility and tribological properties of the coatings. In particular 

the torque-tension characteristics of coated fasteners should be determined under 

repeated loading and unloading conditions. 

Most of the electrical connectors used on military and civil aircraft are cadmium plated. 

The connector shells are normally manufactured from an aluminium alloy, nickel plated 

and then electroplated with cadmium. To date there have been comparatively few 

studies made to identify alternatives to cadmium plating for this application [7]. It is 

recommended that future work under a Garteur collaborative programme should 

examine cadmium substitutes for connectors. 

Page 20 



10 References 

[1]. IATA "Guidance Material on Design and Maintenance against Corrosion 

of Aircraft Structures", DOC.GEN/2637A Issue 2 published by International Air 

Transport Association, Montreal, Canada (1983) 

[2]. "Design requirements for service aircraft" Defence Standard 00-970, 

Ministry of Defence, United Kingdom. 

[3]. 10 th Amendment, Directive 91/338/EEC 

[4]. G Vaessen, F Andrews, C Brindle, E Hultgren, E Kock, D Marchandise, 

W t’Hart and C J E Smith, “Cadmium Substitution on Aircraft”, AGARD Report-R- 

816 Environmentally Compliant Surface Treatments of Materials for Aerospace 

Applications pp15-1 - 15-5 (1997) 

[5]. G Chawa, G D Wilcox, D R Gabe and G M Treacy, "The 

electrodeposition of compositionally multilayer coatings", presented at the IOM 

Materials Congress 2000 - Materials for the 21 st Century, Cirencester,UK (2000) 

[6]. K R Baldwin, R I Bates, R D Arnell and C J E Smith, “Aluminium - 

magnesium Corrosion Resistant Coatings”, published in “Advances in Surface 

Engineering, Volume I: Fundamentals of Coatings Section 1.2 Aqueous Corrosion” 

pp 117-131 

[7]. K R Baldwin, C J E Smith, R I Bates and R D Arnell, "The Corrosion 

Protection of Steel by Aluminium - Magnesium Alloy Coatings", Published in the 

Proceedings of EUROCORR 2000 - Past Success - Future Challenges, IoM 

Communications, London (2000) 

[8]. Air Force Research Lab., "Replacement coatings for aircraft electrical 

connectors: Findings and potential alternatives", US Department of Commerce, 

Tyndall Air Force Base (1998) 



11 Tables 

Coating type 

Commercial identity/ 

Coating specification 

Method of application 

Zinc-cobalt-iron Zincrolyte NCF CF Electrodeposition 

Zinc-nickel Corroban ® Electrodeposition 

Aluminium particles in an 

inorganic matrix 

Aluminium and zinc flakes 

in an inorganic matrix 

SermeTel CR984/985 

Delta-tone + Delta-seal 

Spray 

Electrostatic spraying, 

dipping or spin coating 

Aluminium Galvano-Aluminium Electrodeposited from an 

organic bath 

Aluminium Ivadizer Physical vapour deposition 

Aluminium - magnesium 

Experimental coatings 

DERA/Salford University 

Unbalanced magnetron 

sputtering 

Cadmium Defence Standard 03-19/2 Electrodeposition 

Table 1; Summary of coatings evaluated 

Page 22 



Annex 

Coating characteristics 

Corrosion resistance 

Galvanic compatibility 

Effects of coatings on fatigue strength 

Stress corrosion cracking 

Resistance to aircraft fluids 

Paint adhesion 

Tribological properties 

Coating repair 

- microstructure and composition 

- electrical conductivity 

- coating adhesion 

- barrier properties 

- sacrificial properties 

- torque - tension behaviour 

- coefficient of friction 

B 

C 

D 

E 

F 

G 

H 

I 

J 

Table2; Summary of coating properties and characteristics investigated 



Coating 

Thickness 

μm 

Composition 

ED Cadmium 15 – 20 100% Cd 

Microstructure 

ED Zinc-Nickel 5 – 17 Zn + 9.9%Ni Some cracks visible 

ED Zinc-Cobalt-Iron 10- 18 Zn + 0.8%Co 

Fe trace 

Columnar microstructure 

without cracks 

Delta-tone + Delta-seal 18.2 83% Zn + 2.2% 

Al 

PVD Aluminium 22.6 100% Al Columnar structure 

ED Aluminium 14.1 100% Al Dense featureless coating 

UMS Aluminium- 

10.2 Al + 10%Mg Dense featureless coating 

Magnesium 

SermeTel CR984 35-43 90% Al + 

5% Cr + 5% P 

Table 3; Coating compositions and microstructures 

Electrical resistance [mΩ] 

Coating Hi-Lok fastener Countersink screw 

Type A Type B Type A Type B 

ED Cadmium 0.25 0.17 * 0.19 

ED Zinc-Nickel 1.27 0.99 9.9 0.15 

ED Zinc-Cobalt-Iron 0.72 0.52 * 0.12 

Delta-tone * 87 * 29.3 

Delta-tone + Delta-seal * 21.6 * 19.2 

PVD Aluminium * 2.21 * 0.15 

ED Aluminium NA NA NA NA 

UMS Aluminium-Magnesium NA NA NA NA 

SermeTel CR984 NA NA NA NA 

* values > 1000 mΩ obtained 

NA Measurements not made 

Table 4; Electrical resistance measurements for joints formed using 

Hi-Lok fasteners and countersink screws 

Page 24 



Coating Unpassivated Passivated 

ED Cadmium 45 1 

ED Zinc-Nickel 63 0.5 

ED Zinc-Cobalt-Iron 89 0.6 

Delta-tone 60 - 

PVD Aluminium 2 0.3 

ED Aluminium 0.003 0.006 

UMS Aluminium-Magnesium 0.01 1.3 

SermeTel CR984 0.3 0.8 

Table 5; Corrosion current (μA/cm 2 ) for coatings immersed in 

600mmol/l sodium chloride solution 

Rating As Plated Passivated 

DERA NLR DERA NLR 

Best 

SermeTel CR984 SermeTel CR984 SermeTel 

CR984/985 

ED cadmium 

ED cadmium ED cadmium ED zinc-cobalt-iron SermeTel 

CR984/985 

ED zinc-cobalt-iron ED zinc-cobalt-iron ED aluminium ED zinc-nickel 

ED zinc-nickel ED zinc-nickel ED cadmium ED zinc-cobalt-iron 

ED aluminium 

UMS Al-Mg 

ED zinc-nickel 

PVD aluminium 

Worst 

PVD aluminium 

Table 6; Performance rating of coated panels exposed in the MASTMAASIS test 



As - plated 

Passivated 

DERA Fokker Shorts DERA Fokker Shorts 

Best 

Delta-tone 

ED cadmium 

SermeTel 

CR984 

ED cadmium 

SermeTel 

CR984 

ED cadmium 

ED cadmium 

ED Zn-Ni 

ED Zn-Co-Fe 

ED Zn-Ni 

SermeTel 

CR984/985 

ED cadmium Delta-tone ED Zn-Co-Fe SermeTel 

CR984/985 

ED cadmium 

UMS Al-Mg ED Zn-Ni ED Zn-Ni UMS Al-Mg 

ED Zn-Co-Fe ED Zn-Co-Fe ED Zn-Ni 

ED Zn-Ni 

SermeTel 

CR984 

ED Al 

PVD Al 

ED Al 

PVD Al 

Worst 

SermeTel 

CR984/985 

Table 7; Relative performance of scribed panels exposed to neutral salt fog 

Page 26 



As plated 

Passivated 

Best 

DERA-Fraser Schiphol Warton DERA-Fraser Schiphol Warton 

ED Zn-Co-Fe 

ED Zn-Co-Fe 

ED cadmium 

Delta-tone 

ED Zn-Co-Fe 

ED cadmium 

ED Zn-Co-Fe 

ED cadmium 

ED Zn-Co-Fe 

ED cadmium 

ED Zn-Co-Fe 

ED cadmium 

ED Zn-Ni 

SermeTel 

ED Zn-Ni ED cadmium ED Zn-Ni ED Zn-Ni 

Delta-tone 

SermeTel 

CR984 

ED Zn-Ni 

SermeTel 

CR984/985 

SermeTel 

CR984/985 

ED Zn-Ni 

PVD Al 

SermeTel 

CR984 

SermeTel 

CR984/985 

Worst 

PVD Al 

ED Al 

ED Al 

Table 8; Relative performances of scribed panels in outdoor exposure trials 

DBAA 

Fokker 

Coating As-plated Passivated 

Delta-tone No rusting No rusting 

ED Zn-Co-Fe Rusting after 1000h No corrosion 

PVD Al Rusting after 1000h No rusting 

ED Zn-Ni Rusting after 335h No rusting No corrosion 

ED cadmium Rusting after 335h Rusting after 335h Corrosion in countersink 

Table 9; Corrosion behaviour of bolt/block specimens exposed to neutral salt spray 



As plated 

Passivated 

2014-T6 7075-T6 2014-T6 7075-T6 

Corrosion rate of 

aluminium alloy 

increased above that 

found for cadmium 

plating 

Delta-tone 

UMS Al-Mg 

PVD Al 

SermeTel 

ED Zn-Co-Fe 

Delta-tone 

UMS Al-Mg 

PVD Al 

SermeTel 

ED Zn-Co-Fe 

UMS Al-Mg 

ED Zn-Co-Fe 

PVD Al 

SermeTel 

UMS Al-Mg 

Corrosion rate of 

aluminium alloy similar 

or less than that for 

cadmium plating 

ED Al 

ED Zn-Ni 

ED Al 

ED Zn-Ni 

SermeTel 

PVD Al 

ED Al 

ED Zn-Ni 

ED Al 

ED Zn-Ni 

Table 10; Effect of coating on corrosion rate of aluminium alloy 

ED zinc - nickel 

Coating 

Effect on fatigue strength 

25% reduction 

ED zinc - cobalt, ED cadmium 

10% reduction 

ED aluminium, SermeTel, 

Delta-tone, UBMS Al-Mg 

~ 5% reduction 

Table 11; Effect of coatings on fatigue strength 

Page 28 



Coating 

Aircraft fluids 

1 2 3 4 5 6 7 8 9 

ED zinc-nickel 4 4 4 4 ? 4 7 4 4 

ED zinc-cobalt-iron 4 4 4 4 ? 4 7 4 7 

Delta-tone 4 4 4 4 

PVD aluminium 4 7 

ED aluminium 4 4 4 4 

SermeTel 4 4 4 4 4 4 4 4 4 

ED cadmium 4 4 4 4 4 4 7 4 7 

4 pass 1 Propan-2-ol 2 hydraulic oil - 

OM15 

3 Fuel - Avtur 

7 fail 4 DERD 2497 5 Water (40 o C) 6 Ethylene Glycol 

? marginal 7 Turco 5948 8 Jet A1 9 Skydrol 

Table 12; Susceptibility of coatings to attack by aircraft fluids 



Coating 

Potential risks 

ED Zn-Co-Fe 

ED Zn-Ni 

No known hazard for non-passivated parts 

Chromated surfaces may cause allergy reactions 



SermeTel CR984/985 None - chromium content is 5% 

Chromium is stable and in trivalent condition 

Delta-tone/Delta-seal 

IVD Al 

UMS Al-Mg 

ED Al 

Cadmium 

Comparable to zinc coatings, but no chromated 

surfaces 







General handling of plated parts- if handled with gloves 

provides little or no hazardous effect. 

Direct contact with skin of chromated parts may cause 

allergy reactions 

Table 13; Effect on workers in assembly and maintenance: 

General handling of coated parts 

Page 30 



Coating 


ED Zn-Co-Fe 

ED Zn-Ni 



IVD Al 

UMS Al-Mg 

ED Al 


Alkaline reaction in contact with skin 

No hazard using normal handling procedures 

None, provided that usual precautions are taken 

Application in closed equipment. Evaporation of butanol 


Coating 

Potential costs 

ED Zn-Co-Fe 

ED Zn-Ni 



IVD Al 

UMS Al-Mg 

ED Al 


lower than cadmium 

In line with normal acid (special) wastes 

Accurate cost is unknown, but is minimal 

Only at cleaning process of equipment waste will be 

generated. Handle like zinc 



No waste material; toluene bath is continuously filtered 

Costs of effluent treatment system 

Cost of removal of sludge residues from the process 

tank 

Table 15; Effect on the environment: Costs of disposal of waste materials 

Coating 


ED Zn-Co-Fe 

ED Zn-Ni 



IVD Al 

UMS Al-Mg 

ED Al 


No known effect 

Not highly toxic 

None 

Like zinc coatings 



No risk - aluminium oxide is not considered toxic 

The remnants of cadmium corroded parts at BAe are 

maintained at a minimum, because production 

components can be reworked. Influence of cadmium 

oxide on the environment 

Table 16; Effect on the environment: Remnants of corroded coatings 

Page 32 



Coating 


ED Zn-Co-Fe 

ED Zn-Ni 



IVD Al 

UMS Al-Mg 

ED Al 


Standard neutralisation 

Trace metals contained by effluent plant 

Minimal effect. Some chromium present 

No effect 

No waste water 



If cadmium is allowed to pollute the waste water flow, 

this can lead through the food chain to carcinogenic 

effects on human beings 

Table 17; Effect on the environment: Waste water flow 

Coating 


ED Zn-Co-Fe 

ED Zn-Ni 



IVD Al 

UMS Al-Mg 

ED Al 


No effect 

None 

None 

Butanol 

None 

None 

In the event of leak of the cell, air pollution of toluene is 

possible 

Air pollution can result in long term pulmonary effects 

on animal life and human beings 

Table 18; Effect on the environment: Air pollution 



Properties 

ED 

Zn-Co 

ED 

Zn-Ni 

Serme 

-Tel 

Coating 

Deltatone 

ED Al 

PVD Al 

Conduct. 4 4 4 4 4 

Characteristics Adhesion 4 4 4 4 4 4 

Passivation 4 

Corrosion Barrier 4 4 4 4 4 4 

resistance Sacrificial 4 4 4 4 

Galvanic 4 4 4 4 4 

Fatigue 4 4 4 4 4 

Hydrogen embrittlement 4 4 4 4 

Resist. to aircraft chemicals 4 4 4 4 4 

Tribological Torq.-Ten. 4 4 NT 4 4 4 

properties C.of friction NT 4 

Coating repair 4 4 NT NT NT 4 

Environmental 4 4 4 4 4 4 

Coating Cost 4 4 4 4 

acceptable 4 not acceptable 

not tested 

NT 

Table 20; Summary of properties of commercial coatings compared with cadmium plating 

Page 34 



11 Figures 

Figure 1; Open circuit potentials in 600mmol/litre sodium 

chloride solution 

Figure 2; Average galvanic current determined from 

scratch model specimens exposed to 

600mmol/litre sodium chloride solution 



Figure 3; Protection distances for various metal coatings 

Figure 4; Undamaged panels exposed to neutral salt fog - 

time to red rust 

Page 36 



Figure 5; Undamaged panels exposed to neutral salt fog - 

normalised times to red rust 



This page is intentionally left blank 

Page 38 



ANNEX A 

Coatings and methods of coating application 

A.1 Introduction 

With the exception of the aluminium-magnesium coatings prepared by unbalanced 

magnetron sputtering, all the coatings investigated were deposited using commercially 

available processes. Table A1 identifies the various processes examined and the 

companies where the coating was carried out. More detailed information concerning the 

coating process schedules and standards is given in the subsequent sections of this 

Annex. 

A.2 Coating preparation 

A.2.1 

Electrodeposited zinc-cobalt-iron coatings 

The zinc-cobalt -iron coatings were plated from a non-cyanide alkaline solution supplied 

by Enthone OMI under the tradename Zincrolyte NCZ CF. The chemical composition of 

the bath is given in table A2. Details of the plating schedule are given in table A3. 

A.2.2 

Electrodeposited zinc-nickel coatings 

The electrodeposited zinc-nickel coatings were prepared using the CorroBan ® process. 

The process was originally developed by Boeing and produces a coating containing 

between 6 and 20% nickel. The composition of the electroplating bath employed is given 

in table A4. 

The pH of the bath was maintained in the range 6.1 to 6.3 and the bath temperature held 

between 18 to 26 o C. The coating deposition rate is approximately 10µm in 30 minutes at 

a cathode current density of 54 - 162 amps/m 2 . 

Details of the standard process sequence are given in table A5. 

a) Test panels 

Steel test panels were plated to give a coating thickness of 15 to 20µm. Two sets 

of panels were prepared, one set was left as-plated and the second set was 

treated using the CorroBan ® passivation process. 

b) Fatigue specimens 

Fatigue specimens were prepared in accordance with the schedule given in table 

A6. 

A.2.3 


The SermeTel coatings were applied by Sermatech International Inc. Details of the 

coating procedures employed are given in table A7. 



A.2.4 

Delta-tone/Delta seal 

Coatings were applied by Ewald Dörken AG but detailed information about the process is 

not available. The complete protective scheme, Delta-Magni, consists of two coatings; a 

thin layer containing zinc flakes known as Delta-Tone and an organic coating 

incorporating a lubricant and marketed as Delta-Seal. Literature produced by the 

company indicates that the coatings are applied by dip-spin, dip drain or spray. After 

application the Delta-Tone coating is cured at a temperature of approximately 200 o C. To 

increase the corrosion resistance of the coating and improve the frictional properties a 

coating of Delta-Seal is applied. This is an organic topcoat, which contains a lubricant 

and is also cured at relatively low temperatures. 

A.2.5 

Electrodeposited aluminium 

The electrodeposition of aluminium by the Galvano-Aluminium process is carried out in 

an organic bath consisting of aluminium-alkyl complex solutions. The process allows 

aluminium to be deposited directly onto a steel substrate unlike earlier processes which 

required the use of a copper or nickel intermediate layer. The various stages in the 

plating process are detailed in table A8. 

The bath parameters employed are given in table A9. 

A.2.6 

PVD aluminium coatings 

Aluminium coated panels and fasteners prepared by physical vapour deposition were 

supplied by Aerocoat. The deposition process was carried out in accordance with 

Defence Standard 03-28(Part 1). Details of the procedure used are given in table A10. 

A.2.7 

Unbalanced magnetron sputtered aluminium - magnesium coatings 

Aluminium - magnesium coatings were deposited using an unbalanced magnetron 

sputtering technique operated at Salford University. The test panels were initially 

degreased using an ultrasonic bath containing alcohol and then placed in the vacuum 

chamber. The magnetron sputtering chamber was evacuated to 10 -5 Torr or below and 

then backfilled with high purity argon to a pressure of around 5x10 -3 Torr. The chamber 

contained two consumable targets, one commercial grade aluminium (99.4% purity) and 

one of commercial grade magnesium (99.5% purity) Prior to deposition, the steel test 

panels were sputter cleaned, typically at an applied voltage of 1 kV for 30minutes to 

remove any traces of surface contamination. 

Following this high voltage sputter cleaning process, a low-voltage was applied to the 

test panels, typically -50V (DC) to provide an external bias during deposition. The targets 

were energised using an electrical current to typical power levels of 3kW for an 

aluminium target and 1.5 kW for magnesium target. The coating process was continued 

for a sufficient time to allow the required thickness of deposit to build up on the 

substrates. After the desired thickness had been obtained, the deposition process was 

halted and the coupons allowed to cool under vacuum to prevent oxidation. A typical 

deposition rate was 0.3μm min -1 . Further information on the unbalanced magnetron 

sputtering of aluminium - magnesium coatings is given in references A2 and A3. 

A.2.8 

Electrodeposited cadmium coatings 

Panels and fasteners were coated using a standard cyanide electroplating bath operated 

in accordance with Defence Standard 03-19 [A4]. 

Page 40 



A.3 References 

[A1] 

[A2] 

[A3] 

[A4] 

Defence Standard 03-28(PART 1)/Issue 1 “Physical Vapour Deposition of Metals” 

Part 1:Ion Vapour Deposition of Aluminium for Protection against Corrosion”, 

Ministry of Defence, Directorate of Standardization, Kentigern House, 65 Brown 

Street, Glasgow G2 8EX (1988) 

K R Baldwin, R I Bates, R D Arnell and C J E Smith, “Aluminium - magnesium 

alloys as corrosion resistant coatings for steel”, Corrosion Science 38 No.1 pp 

155-170 (1996) 

K R Baldwin, R I Bates, R D Arnell and C J E Smith, “Aluminium - magnesium 

Corrosion Resistant Coatings”, published in “Advances in Surface Engineering, 

Volume I: Fundamentals of Coatings Section 1.2 Aqueous Corrosion” pp 117-131 

(1997) 

Defence Standard 03-19 “Electrodeposition of cadmium”, Ministry of Defence, 

Directorate of Standardization, Kentigern House, 65 Brown Street, Glasgow G2 

8EX (1981) 



A.4 Tables 

Coating type Commercial process Coater Partner Responsible 

Electrodeposited 

Zinc-Cobalt-Iron 

Zincrolyte NCZ CF SAAB SAAB 


Zinc-Nickel 

CorroBan ® 

1) South West Metal 

Finishers 

2) Shorts 

Shorts 

Metallic-ceramic 

SermeTel 

CR984/985 

Sermatech 

International Inc 

Shorts 

Metallic-ceramic DELTA-Tone + 

DELTA-Seal 

Ewald Dörken AG 

Daimler-Benz 

Aerospace Airbus 


aluminium 

Galvano-aluminium 

Alcotec 

Beschichtungsanlag 

en GmbH 

Aerospatiale 

PVD 

coatings 

aluminium 

Carried out to 

Def. Stan 03-28 

part(1) 

Petroplus/ 

Aerocoat, Germany 

DERA 

Unbalanced 

magnetron sputtered 

aluminium - 

magnesium 

Experimental 

process 

Salford University 

DERA 


cadmium 

Carried out to 

Def. Stan 03-19 

British Aerospace 


Table A1; Summary of coating processes evaluated 

Chemical 

Concentration 

(gm/litre) 

Zinc 7.3 

Sodium hydroxide 99 

Cobalt 118 

Iron 39 

Part D 29 

Sodium carbonate 40 

Table A2; Composition of zinc-cobalt-iron (Zincrolyte 

NCZ CF) electroplating bath 

Page 42 



1 Dry grit blast with fine alumina 180-220 mesh 

2 Anodically cleaned 

3 Electroplating in Zincrolyte NCZ CF 

Voltage ~2.8V 

Current density ~2.5A/dm 2 

Plating time 35 minutes 

4 Passivation in Udyfin 7748 

Concentration 30ml/litre 

pH 1.6 

Immersion time 35-40 seconds 

Table A3; Zinc-cobalt-iron electroplating schedule 

Chemical 

Concentration 

(gm/litre) 

Ammonium chloride 171.5 

Nickel chloride 73.8 

Zinc chloride 16.7 

Boric acid 20.3 

BOE-NIZ LHE 30.3 

Table A4; Composition of CorroBan ® zinc-nickel 

electroplating bath 



1 Abrasive blast using aluminium oxide or glass beads 

2 Alkaline clean 

3 Rinse 

4 Hydochloric acid dip 

5 Rinse 

6 CorroBan ® Zinc-Nickel electroplating bath 

7 Rinse 

8 Dry 

9 Hydrogen embrittlement relief bake (if required) within 8 hours 

10 CorroBan ® passivation process (if required) 

11 Rinse 

12 Dry 

Table A5; Zinc-nickel electroplating process sequence 

Page 44 



1 Stress relieve at 420 o C for 1 hour 

2 Mask threads 

3 Abrasive clean using 100 mesh alumina at 25 PSI with 

a nominal “Stand off” (nozzle to part) distance of 125 - 150mm 

4 Alkaline clean for 10-15 minutes in Altrex 1097 and rinse 

5 Hydrochloric acid dip (3%/volume Hcl) for 10-30 seconds 

and rinse 

6 Plate 

7 Rinse 

8 Bake (de-embrittle) within 8 hours at 191 o C + 14 o C 

Table A6; Zinc-nickel electroplating sequence for fatigue specimens 

1 Heat treat at 420 o C for one hour 

2 Mask threaded areas of specimens 

3 Grit blast with 100 mesh alumina at 40 psi 

Nozzle distance nominal 15 - 20cms 

The notches of the specimens were blasted using a pencil blaster with 329 mesh 

alumina. This was necessary to ensure that all areas inside the notch were properly 

blasted. 

4 Re-mask threaded areas 

5 Apply one coat SermeTel 984. Cure at 190 o C for one hour. 

6 Grit burnish with 320 mesh alumina at 20psi 

7 Measure electrical conductivity. Requirement


1 Activation bath (non aqueous) 

2 Rinsing fluid bath 

3 Aluminizing (non aqueous) 

4 Outlet rinsing bath 

5 Activation (aqueous) 

Table A8; Galvano-Aluminium electrodeposited aluminium 

process sequence 

Electrolyte KF[1.5Al Et 3 , 0.25 Al-i-Bu 3 , ).25AlMe].3C 7 H 8 

Coating temperature 

100 o C 

Coating voltage 

5 - 30V 

Current density 0.2 - 2.0 A/dm 2 

Conductivity at 100 o C 

25 mS/cm 

Current characteristic 

Polarity switching, rectangular pulse 

Deposition rate 12μm at 1 A/dm 2 

Current efficiency 

100% at the cathode and at the anode 

Table A9; Electrolyte properties and process parameters 

1 Abrasive cleaning with alumina grit 

2 Parts transferred to coating chamber 

3 Sputter clean 

4 Coat with aluminium 

5 Remove parts from coating chamber 

4 Glass bead peen 

5 Chromate conversion coating 

Table A10; Physical Vapour Deposition sequence 

Page 46 



ANNEX B 

Coating characteristics 

B.1 Introduction 

The coatings evaluated in the Garteur "Cadmium Substitution" programme were either 

supplied by specialist coating firms or were prepared by Partners using processes 

available commercially. 

The main objectives of the characterisation work were:- 

• to measure the thicknesses of the coatings being studied 

• to check the quality of the coatings being deposited by examining the coating 

uniformity, microstructure and composition 

• to check the adhesion of the coating to the steel substrate 

• to determine the electrical conductivity of the coatings 

The overall aim was to confirm that the coatings being evaluated were representative of 

those normally supplied to the aerospace and other manufacturing industries. 

B.2 Test procedures 

A series of tests were conducted to determine the microstructure, composition, electrical 

conductivity and adhesion of the range coatings detailed in table 1. Details of the test 

procedures employed are given in following sections. 

B.2.1 

Microstructure and composition 

Metallographic cross sections were prepared for each of the coatings. These were 

examined using optical microscopy to enable the coating thickness and porosity to be 

determined. In addition information about the surface treatments applied to the steel 

substrates was obtained. 

Several of the coatings were examined using scanning electron microscopy to enable a 

more detailed picture of the microstructure to be formed. 

The chemical composition of each coating was determined by ICP-AES. 

Coating thicknesses were also determined using a non-destructive technique based on 

Eddy current measurements. 

B.2.2 

Electrical conductivity 

Electrical conductivity measurements were made using two simple lap joint specimen 

designs. In the first a 2mm thick aluminium alloy sheet panel was fastened to a 8mm 

thick aluminium alloy plate panel by means of a coated Hi-Lok fastener. In the second 

design two 2mm aluminium alloy sheet panels were joined by means of a coated 

countersink screw. The designs of the lap joint specimens are shown schematically in 

figures B1(a) and B1(b). In each case the aluminium alloy panels were chromic acid 

anodised and painted before assembly. The tightening torque applied to the fasteners 

was 0.22 to 0.26 daNm in accordance with Airbus specifications. For each coating type 

three Hi-Lok and three countersink screw specimens were prepared. 

For each specimen type two sets of resistance measurements were made as follows:- 



Type A: 

Type B: 

electrical resistance across the complete lap joint 

electrical resistance between one end of lap joint 

and the fastener 

The aim of the work was to determine whether a conducting path could be established 

through the coated fastener. 

B.2.3 

Coating adhesion 

B.3 Results 

A low temperature tensile test method was employed by NLR to determine the adhesion 

of the coatings to the steel substrate. Details of the test procedure are given in reference 

[B1]. The tensile specimens used in these tests were tapered to give a gradual plastic 

strain distribution along the specimen axis as a result of tensile testing. After testing 

cross sections provided information on the crack density as a function of increasing 

plastic strain. 

B.3.1 

B.3.1.1 

Microstructure and composition 

Microstructure 

The microstructures of the eight coatings evaluated are shown in figures B2 and B3. 


The topcoat of the SermeTel CR984/985 is cracked. It can be seen that the SermeTel 

coating consists of two layers and the outer layer has been burnished to obtain electrical 

conductivity of the coating. The aluminium particles in the inorganic matrix are visible. 

The SermeTel coating is not completely dense and shows pores, also along the interface 

of the two sprayed layers. 

Electrodeposited zinc-cobalt-iron coatings 

Figure B2 shows the microstructure of the zinc-cobalt-iron coatings. The passivation 

layer is cracked and shows some features, which might originate from the passivation 

process. The cross sections show a columnar microstructure without cracks. 

Electrodeposited zinc-nickel coatings 

Some cracks through the coating down to the steel substrate can be seen. Remnants of 

the grit blasting process using alumina lead to subsurface cracks in the steel. 

Electrodeposited cadmium coatings 

The passivation layer is cracked. The cross sections of non-passivated and passivated 

coatings show different microstructure and the thickness of the non-passivated coating is 

two to three times the thickness of the passivated coating. 

Ion vapour deposited aluminium coatings 

Coatings supplied showed the normal porous structure (figure B3) found with ion vapour 

deposited coatings. Glass bead peening applied after plating tends to densify the coating 

Unbalanced magnetron sputtered aluminium magnesium coatings 

The aluminium coatings produced by unbalanced magnetron sputtering are very dense 

as indicated by the microgaph in figure B2 obtained using scanning electron microscopy. 

Electrodeposited aluminium coatings 

On the examination of cross sections using scanning electron microscopy, the electrodeposited 

aluminium coatings appear extremely dense as illustrated in figure B3. 

Page 48 



Samples prepared to show the appearance of the surface after passivation revealed a 

cracked structure similar to that found after the chromate filming of aluminium alloys. 

Delta-tone/Delta-seal coatings 

The optical micrograph in figure B2 shows a microsection prepared from a sample 

coated with Delta-tone. The microstructure is seen to consist of a matrix containing 

flakes of zinc typically 3-4μm thick and up to 30μm long. 

B.3.1.2 

Composition 

The main constituents of the unpassivated coatings are given in table B1 below. For the 

alloy coatings, it was found that the zinc-nickel alloy contained 10 wt-% nickel whereas 

the zinc-cobalt-iron alloy contained 0.8 wt-% cobalt and a trace of iron (


B.3.3 

Coating Adhesion 

The coating adhesion on the grit blasted steel substrate was good. The metallic coatings 

behave in a brittle manner (except ED cadmium) resulting in a high crack density at a 

plastic strain of 1%. At increasing strain no signs of coating delamination were observed. 

Tests conducted on Delta-tone indicated that the surface roughness is a critical factor in 

determining the coating adhesion. A smooth surface was found to give the best 

adhesion. If the surface roughness is too high, a micro-layer of Delta-tone prevents 

complete wetting of the surface and cavities are formed at the coating/steel substrate 

interface. 

B.4 Conclusions 

1. The microstructures, thicknesses and compositions of the coatings supplied for the 

Garteur programme have been compared with information given by the coating 

producers and with data published in the open literature. It is concluded that 

coatings are representative of those generally used in the aerospace and other 

manufacturing industries. 

2. Resistance measurements indicate that it is possible to achieve good electrical 

conductivity with both coated Hi-Lok and coated countersink screws. Tests made on 

simple aluminium alloy lap joints indicate that there is electrical contact between the 

fasteners and aluminium alloy. 

3. All the metal coatings investigated showed good adhesion to steel substrates which 

had been grit blasted. Tests made on the Delta-tone coating indicated that the best 

adhesion was achieved when the substrate was smooth. 

B.5 References 

[B1] 

W.G.J. t Hart and J.A.M Boogers, “Coatings considered for cadmium substitution 

(Garteur AG17 Programme), NLR CR 97107 L (1997) 

Page 50 



B.6 Tables 

Coating 

Element (concentration – wt%) 

Al Cd Co Cr Fe Mg Ni P Ti Zn 

ED Cadmium 100 

ED Zinc-Nickel 9.9 90.1 

ED Zinc-Cobalt-Iron 0.8 t 99.2 

Delta-tone 2.2 3.1 5.4 83.6 

PVD Aluminium 100 

ED Aluminium 100 

UBMS Aluminium- 90 10 

Magnesium 

SerMetel 984 90 5 5 

Table B1; Chemical composition of Garteur coatings 

Coating 

Eddy Current 

Range 

(μm) 

Average 

(μm) 

Optical 

X-section 

(μm) 

ED Cadmium 20-28 22.5 15-20 

ED Zinc-Nickel 13-17 15.2 5-10 

ED Zinc-Cobalt-Iron 16-19 17.9 10 

Delta-tone 17-32 24.5 10-15 

PVD Aluminium 18-23 21.1 

ED Aluminium 12-16 14.4 

UBMS Aluminium-Magnesium 7-12 9.6 

SermeTel 984 32-48 39.4 35 

Table B2; Coating thickness measurements 



Type A Measurements 

Type B Measurements 

Coating Resistance [mΩ] Average [mΩ] Resistance [mΩ] Average [mΩ] 

0.24 0.16 

ED Cd 0.25 0.25 0.18 0.17 

0.26 0.18 

0.85 0.63 

ED Zn-Ni 2.04 1.27 1.75 0.99 

0.94 0.61 

0.42 0.24 

ED Zn-Co-Fe 0.39 0.72 0.22 0.52 

1.37 1.11 

- - 

Delta-tone 108 74 87 

152 100 

>1000 5.85 

PVD Al 0.49 0.42 2.21 

>1000 0.36 

12.3 1.62 

Delta-tone + seal 10.0 7.21 21.6 

59 56 

Table B3; Electrical Conductivity Measurements – Lap joints formed using a Hi-Lok fastener 

Type A measurements 

Type B measurements 

Coating Resistance [mΩ] Average [mΩ] Resistance [mΩ] Average [mΩ] 

1.92 0.12 

ED Cd >1000 0.12 0.19 

1.41 0.34 

2.83 0.15 

ED Zn-Ni 22.4 9.9 0.14 0.15 

4.7 0.17 

0.84 0.12 

ED Zn-Co-Fe >1000 0.12 0.12 

>1000 0.13 

>1000 24.2 

Delta-tone >1000 47.6 29.3 

98.5 40.2 

>1000 0.15 

PVD Al >1000 0.14 0.15 

3.98 0.15 

37.2 25.0 

Delta-tone + seal >1000 13.9 19.2 

>1000 18.6 

Table B4; Electrical Conductivity Measurements – Lap joints formed using a countersink screw 

Page 52 



Strain % 

Coating 0.5 1 3.5 5 7.5 8 14 

ED cadmium 0 0 0 0 

ED Zinc-cobalt-iron 25 32 34 

ED Zinc-nickel 16 20 22 21 

SermeTel 984 12 12 11 12 13 

SermeTel 984/985 8 7 11 11 

Table B5; Crack density in coatings for different plastic deformations (cracks/mm) 



B.7 Figures 

Hi - Lok 

← 25mm → 

.07mm diameter 

Side View 

8mm 

2mm 

← 25mm → 

30mm 

Plan View 

Figure B1(a); Schematic diagram showing construction of a simple lap joint specimen 

using a Hi-Lok fastener 

Countersink screw 

4.2mm diameter 

Side View 

2mm 

2mm 

30mm 

Plan View 

Figure B1(b) Schematic diagram showing construction of a simple lap joint specimen 

using a countersink screw 

Page 54 



ED Zinc-nickel 

Delta-tone 

ED Zinc-cobalt-iron coating 

SermeTel 984 + SermeTel 985 coating 

ED Cadmium 

Figure B2; Optical micrographs showing structures of zinc alloy, cadmium and metallic - 

ceramic coatings 



ED Cadmium 

IVD Aluminium 

ED Aluminium 

UBMS Aluminium - magnesium 

Figure B3; SEM micrographs showing structure of aluminium coatings 

Page 56 



ANNEX C 

Corrosion resistance 

C.1 Introduction 

Two aspects of the corrosion protection afforded by replacement metal coatings were 

investigated, 

• performance as a barrier coating 

• sacrificial properties of the coating 

The range of tests employed and the types of test specimen used are summarised in 

table C1. 

The accelerated tests and outdoor exposure trials conducted on undamaged panels 

were aimed at establishing the overall corrosion resistance of the coatings. The 

appearance of red rust was used as one of the criteria in comparing the performance of 

the various coatings. This was taken as an indication of the barrier properties of the 

coatings, although once the coating has corroded through to the substrate the sacrificial 

properties of the coating will be important. Scribed panels were used to semi-quantify the 

relative sacrificial properties of the coatings. 

Electrochemical measurements enable a more detailed understanding of the barrier and 

sacrificial properties of the coatings in saline environments to be established. The 

techniques employed allow the corrosion resistance of the coatings to be measured and 

the galvanic corrosion current and protection distances to be determined. 

C.2 Test procedures 

Details of the accelerated test procedures and outdoor exposure trials are given below 

together with the electrochemical techniques employed. 

C.2.1 

C.2.1.1 

Accelerated corrosion tests 

Exposure to 5% neutral salt fog 

Testing was carried out in accordance with ASTM B117 [C1]. Coated test panels were 

exposed to a continuous 5% neutral salt spray for up to six weeks in a cabinet 

maintained at a temperature of 35 o C. The panels were inspected at regular intervals for 

the first signs of red rust. 

C.2.1.2 

Exposure to intermittent acidified salt spray (MASTMAASIS) 

Coated test panels were assessed using an intermittent acidified salt spray test based on 

the procedure described by Lifka and Sprowls [C2] and covered by ASTM G85-84 [C3]. 

The test was originally developed for evaluating the susceptibility of aerospace 

aluminium alloys to exfoliation corrosion but has found wider applications in the study of 

metal coatings. 

In the present work coated panels were exposed for periods of up to 3000hours to 

repeated 6 hour test cycles. Each test cycle consisted of a 45 minute spray with 5% 

acidified salt solution, a 2 hour direct air purge into the roof of the cabinet during which 

the relative humidity fell to between 40 and 45%, followed by a 3 hour 15 minute soak 

when the relative humidity rose to 90 to 95%. In tests conducted at DERA the acidity of 



the salt spray was adjusted to pH 3 with acetic acid whilst at NLR the pH of the test 

solution used was kept at pH 5. 

C.2.1.3 

Exposure to 100% humidity 

Coated test panels were exposed to 100% humidity according to ASTM D2247 [C4]. The 

temperature of the saturated air in the cabinet was maintained at 38+1 o C throughout the 

2000 hours exposure period. The test panels were inspected on a regular basis for signs 

of white rust and the onset of red rust. 

C.2.2 

Outdoor exposure trials 

Outdoor exposure trials on coated panels were carried out at four sites as follows:- 

• DERA Fraser, Portsmouth, England 

• British Aerospace, Warton, Preston England 

• Fokker, Schiphol, The Netherlands 

• NLR, NE Polder, The Netherlands 

At each site specimens were placed on racks inclined at an angle of 45 o to the horizontal 

and facing due south. The sky ward faces of the panels were inspected at least once a 

month for signs of corrosion and general degradation. 

Two of the sites, DERA Fraser and NLR NE Polder, are close to open sea whilst the 

British Aerospace site is some distance from sea. The Fokker site is in land adjacent to 

Schiphol Airport. 

C.2.3 

C.2.3.1 

Electrochemical measurements 

Polarisation sweeps 

Anodic and cathodic polarisation sweeps were carried out on small test electrodes cut 

from the coated panels. Details of the manufacture of the test electrodes used at DERA 

are given in reference [C5] together with the design of the test cell and the scan rate etc. 

employed. Research at NLR used a modified flat cell and further details of the test 

procedure are described in reference [C6]. 

C.2.3.2 

Potential-time measurements 

The open-circuit potential of each coating system was determined by immersing small 

test electrodes prepared from coated test panels in quiescent sodium chloride solution. 

The change in open circuit corrosion potential was monitored over a period of several 

hours using a reference electrode. The laboratories carrying out these measurements 

looked at the effects of sodium chloride concentration and pH on the potential. More 

detailed information is included in references [C6,C7 and C8]. 

C.2.3.3 

Scratch-Model electrochemical measurements 

The Scratch-Model approach was developed at DERA to model the situation that may 

arise in a coating scratch, where a small area of the steel becomes exposed to the 

environment. Precise details of the specimen construction have been published in the 

literature [C9]. The experimental arrangement employed in the present work is illustrated 

in figure C1. Briefly a steel strip (1mm x 20mm) was positioned on a glass slide. On 

either side of this was positioned coated, steel coupons (20mmx20mm). The assembly 

Page 58 



was then immersed in 600 mmol l -1 sodium chloride solution at 25 o C for a period of two 

weeks, during which time the galvanic current was monitored using a zero resistance 

ammeter and chart recorder. 

C.2.3.4 

Cathodic protection measurement technique 

C.3 Results 

The technique involves the measurement of potentials along a steel strip immersed in 6 

mmol l -1 sodium chloride solution at 25+ 2 o C. The coating of interest is attached at one 

end of the strip as shown in figure C2. The potential along the strip is measured via a 

series of 11 saturated calomel electrodes each independently connected to a data 

logger. The SCEs were at the following distances from the edge of the coating; 

0, 5, 10, 17, 22, 28, 40, 60, 90, 120, 150mm 

Potential measurements along the steel strips were carried out for up to 1500 hours. 

Further details of the technique are given at reference C10. 

C.3.1 

C.3.1.1 

Barrier properties 


a) Resistance to neutral salt fog 

Neutral salt spray tests were conducted by three laboratories DERA, Shorts and Fokker 

and the results in terms of the time to red rust are given in Table C5. 

Although the actual times to red rust for undamaged panels varied considerably between 

the three laboratories, there was general agreement regarding the relative performances 

of the zinc based coatings and the SermeTel coating. The data show that only the 

metallic ceramic coatings, SermeTel out performed cadmium when tested in the as 

plated condition. In every case carrying out post plating treatments such as passivating 

considerably improved the performance of the coating but only the SermeTel 984 coating 

with SermeTel 985 gave a higher time to first red rust than passivated cadmium plating. 

As plated 

SermeTel 984 > ED cadmium >ED zinc – nickel ~ ED zinc – cobalt – iron 

Plated + post plating treatment 

SermeTel 984 + 985 > Pass. ED cadmium > Pass. ED Zn-Ni ~ Pass. ED Zn-Co-Fe 

Tests on aluminium and aluminium based coatings and Magnicoat were only carried out 

at DERA. From Table C5 it is apparent that whilst the as plated UMS Al-Mg, Magnicoat 

and SermeTel coatings out perform cadmium plating in the neutral salt fog tests, the 

passivated cadmium plating gives the highest level of corrosion protection. 

b) Exposure to intermittent acidified salt spray (MASTMAASIS) 

MASTMAASIS testing was undertaken by DERA and NLR and the results are 

summarised in table C6. Whilst good agreement was obtained for tests conducted on as 

plated panels, some differences were found on tests carried out on passivated panels. 



c) Exposure to 100% humidity 

The exposure of panels plated with cadmium, zinc alloys and the two metallic ceramic 

coatings Delta-tone and SermeTel to 100% humidity was investigated by Fokker. The 

tests were terminated after 2000 hours and the results obtained are presented in table 

C7. Rust spots were only found on the Delta-tone sample, all the remaining test panels 

survived the 2000 hour exposure period. 

C.3.1.2 

Outdoor exposure 

Results of the outdoor exposure trials on non-scribed panels are given in table C5. Trials 

and Schiphol were terminated after 462 days. During that time only the electrodeposited 

zinc-nickel coating showed evidence of red rust. Tests conducted at the DERA Fraser 

marine exposure site were continued for two years. During that period evidence of 

rusting was found on all the aluminium coatings and the non-passivated zinc-nickel 

coating. The two metallic-ceramic coatings, Delta-tone and SermeTel, and the 

electrodeposited zinc-cobalt-iron coatings showed no evidence of rusting. 

C.3.1.3 


The polarisation studies were used to estimate the corrosion current density for each of 

the coatings in sodium chloride solution. The results obtained are presented in table C6. 

As found in the accelerated tests, passivation treatments generally decreased the 

corrosion rate of the coating. The aluminium based coatings had lower corrosion rates 

than the zinc rich deposits suggesting they possessed better barrier properties. 

C.3.2 

Sacrificial properties 

Accelerated corrosion tests and outdoor exposure trials on scribed panels were used to 

compare the relative sacrificial properties of the coatings. The time to the appearance of 

red rust in the scribe was used as a measure of the coatings ability to cathodically 

protect the substrate at areas where the coating is damaged. 

C.3.2.1 


Results of the neutral salt fog, MASTMAASIS and 100% humidity tests are given in 

tables C7, C8 and C9 respectively. The data refer to the occurrence of red rust within 

thescratch region. In the case of the zinc rich coatings, the time to red rust was 

comparable with cadmium plating. However the aluminium coatings generally showed 

relatively poor performance, with red rust being detected in the salt spray test after 

relatively short exposure periods. Variable results were obtained with the SermeTel 

coatings. These were considerably thicker than the zinc alloy and aluminium alloy 

coatings and may have had an influence on the results. 

C.3.2.2 


Outdoor exposure data obtained from four test sites are summarised in table C10. The 

results indicate that electrodeposited zinc-nickel coatings are less effective than 

cadmium plating in protecting the scribed area. The zinc-cobalt-iron system appears very 

similar to cadmium in performance in both marine and relatively rural environments The 

aluminium based schemes including SermeTel 984 were the least efficient coatings and 

in the most favourable case did not delay the onset of rusting for more than six months. 

This suggests that in a marine environment, sacrificial coatings are more effective than 

simple barrier coatings. 

Page 60 



C.3.2.3 


Results of the OCP measurements are presented in table C11. In general the zinc based 

coatings (ED zinc-nickel, ED zinc-cobalt-iron and Delta-tone) exhibit much lower 

potentials than the aluminium coatings. On this basis it would be expected that the zinc 

rich coatings would provide a greater level of sacrificial protection and this is supported 

by the accelerated corrosion tests and outdoor exposure trials on scribed panels. 

The average galvanic corrosion currents measured using the Scratch Model specimens 

are summarised in table C12. In broad agreement with the findings of the OCP studies 

the as deposited zinc based coatings show higher levels of galvanic corrosion current 

than the pure aluminium coatings and SermeTel 984. Measurements made on the UBMS 

aluminium - magnesium coating confirm that this is a more sacrificial coating than pure 

aluminium. 

Data obtained from the cathodic protection measurement technique were used to 

calculate the protection distance for each of the coatings studied. This represents the 

distance over which a coating can provide sacrificial protection to the steel strip. The 

results are presented in table C13. This shows that the two pure aluminium coatings are 

relatively poor sacrificial coatings whilst the zinc based coatings provide protection over 

much greater distances than cadmium plating. The aluminium - magnesium coating 

shows a significant improvement over pure aluminium and is more effective than 

cadmium plating. 

C.4 Conclusions 

1. Overall the aluminium based coatings were found to have superior barrier properties 

to the zinc rich deposits studied in this investigation. 

2. Electrochemical and accelerated corrosion tests have demonstrated that the zinc rich 

deposits are more effective in preventing the onset of rusting at scratches or defects 

in the coating than pure aluminium or aluminium alloy coatings. 

C.5 References 

[C1] 

[C2] 

[C3] 

Standard method of salt spray (fog) testing, ASTM B117-73 published by the 

American Society for Testing and Materials, Philadelphia, Pa. 19103 (1973) 

Lifka B.W. and Sprowls D. "An improved exfoliation test for aluminium alloys", 

Corrosion 22 p7 (1966) 

Modified salt spray (fog) testing, ASTM G85-84 published by the American 

Society for Testing and Materials, Philadelphia, Pa. 19103 (1984) 

[C4] Standard method for Testing Coated Metal Specimens at 100% Relative 

Humidity, ASTM D2247-68 published by the American Society for Testing and 

Materials, Philadelphia, Pa. 19103 (1980) 

[C5] 

[C6] 

Smith C.J.E., Baldwin K.R., Hewins M.A.H. and Gibson M.C., “A Study into the 

Corrosion Inhibition of an Aluminium Alloy by Cerium Salts” published in 

“Progress in the Understanding and Prevention of Corrosion” pp1652 - 1663 

(1993) 

Jansen E.F.M. and 'tHart W.G.J., Corrosion properties of cadmium, ZnNi, ZnCoFe 

and SERMETEL coatings - Garteur Cadmium Substitution programme NLR 

Progress Report 1- NLR CR 95607 L (1995) 



[C7] Baldwin K.R., Alternatives to cadmium plating, DRA/SMC/CR971021, (1997) 

[C8] Morgan P.C., Garteur Cadmium Replacement Programme, Sowerby Research 

Centre Ref:755175/256/2880/PCM (1996) 

[C9] 

Baldwin K.R., Robinson M.J. and Smith C.J.E. The galvanic corrosion beahviour 

of electrodeposited Zn-Ni alloy coatings coupled with steel, British Corrosion 

Journal 29(4) p293 (1994) 

[C10] Baldwin K.R., Smith C.J.E. and Robinson M.J., “Cathodic protection of steel by 

electrodeposited zinc-nickel alloy coatings” Corrosion 51 No.12 pp932-940 (1995) 

Page 62 



C.6 Tables 

Test 

Barrier 

Properties 

Sacrificial 

Properties 

Neutral salt fog undamaged panels scribed panels 

Accelerated 

Corrosion Tests MASTMAASIS undamaged panels scribed panels 

100% humidity undamaged panels scribed panels 

Outdoor 

exposure undamaged panels scribed panels 

Open circuit potential 

measurements 

Electrochemical polarisation sweeps 

measurements scratch model specimens 

potential drop measurements 

Table C1; Summary of corrosion tests to determine barrier and sacrificial properties of coatings 

Post plating treatment 

Coating Test Unpassivated Passivated 

Location Thick(μm) Time(h) Thick(μm) Time(h) 

Shorts 2523h >3723h 

ED Cadmium Fokker 15-20 >2000h 7 >2000h 

DERA 18.4 792h 15.4 2400h 

Shorts 368-528h >3723h 

ED Zinc-nickel Fokker 5-10 270, >2000 10-12 >2000h 

DERA 16.9 440h 14.2 1080h 

ED Zinc-cobaltiron 

Fokker 10 432,672, 10-12 >2000 

600, 672h 

DERA 18.1 310 16.8 1320 

PVD Aluminium DERA 22.6 350h 29.2 1576h 

ED Aluminium DERA 14.1 216 17.3 1190 

UBMS Almagnesium 

DERA 10.2 710 11.1 1360 

Delta-tone/Deltaseal 

Delta-tone 

DERA 18.2 2056 


(984) (984/985) 

Shorts >3723 >3723 

SermeTel Fokker 35 >2000 40 >2000 

DERA 43.2 3020 36.7 5680 

Table C2; Neutral salt spray (ASTM B117) - panels not scribed 

Time to red 



Coating 

rust (hours) 

ED Cadmium 1368, 1608 

ED Cadmium passivated 1752, 1392 

ED Zinc-nickel 576, 552 

ED Zinc-nickel passivated 552, 1992 

ED Zinc-cobalt-iron 840, 936 

ED Zinc-cobalt-iron passivated 2496, 2088 

PVD Aluminium 108, 108 

PVD Aluminium passivated 360, 360 

ED Aluminium 240 

ED Aluminium passivated 1448 

Aluminium- 

UBMS 

magnesium 

216 

Delta-tone 

Delta-tone + Delta-seal 

Sermetel 984 >3000 

Sermetel 984/985 >3000 

Table C3; Summary of MASTMAASIS test results - 

panels not scribed 

Coating 

Unpassivated 


Passivated 

Thick(μm) Time(h) Thick(μm) Time(h) 

ED Cadmium 15-20 >2000 7 >2000 

no rust 

no rust 

ED Zinc-nickel 5-10 >2000 10-12 >2000 

no rust 

no rust 

ED Zinc-cobalt-iron 10 >2000 10-12 >2000 

no rust 

no rust 

Delta-tone 


Delta-tone/Delta-seal 18.2 1080 

rust spots 

(984) (984/985) 

SermeTel 35 >2000 40 >2000 

no rust 

no rust 

Table C4; 100% humidity (ASTM D2247) - panels not scribed 

Page 64 



Coating 

Exposure Site 

DERA Schiphol 

Cadmium not passivated >536 >462 

passivated >536 >462 

ED Zinc-nickel not passivated 385 213 


ED Zinc-cobaltiron 

not passivated >659 >462 


IVD Aluminium not passivated 284 

passivated 455 

ED Aluminium not passivated 85 

passivated 62 

UMS Aluminium not passivated 300 

- magnesium 

Delta-tone >536 >462 

SermeTel 984 >659 >462 

SermeTel 984 / 985 >659 >462 

Table C5; Outdoor exposure trials on non-scribed panels - 

Time to red rust (days) 

Coating 

DERA NLR NLR B Ae 

600mmol l -1 100mmol l -1 100mmol l -1 514mmol l -1 

NaCl 

NaCl pH 2 

NaCl 

NaCl 

pass pass pass pass 

ED Cadmium 45 1 78 60 63 24 25-50 0.33- 

2.1 

ED Zinc-nickel 63 0.5 97-187 41-232 4-14 1-2 0.4-50 15 

ED Zinc-cobalt-iron 89 0.6 233- 

477 

4 0.2 14-22 0.04- 

013 

PVD Aluminium 2 0.3 


UBMS Al-Mg 0.01 1.3 

Delta-tone 60 - 

SermeTel 984 0.3 20 0.5 0.17 

SermeTel 984/985 0.8 9.2 0.4 1.12 

Table C6; Corrosion current (μA/cm 2 ) derived from polarisation measurements 




Coating Test Unpassivated Passivated 

Location Thick(μm) Time(h) Thick(μm) Time(h) 

Shorts >3723 2115 

ED Cadmium Fokker 15 - 20 >2000 7 >2000 

DERA 24.0 690 15.6 1968 

Shorts 368 - 528 >3723 

ED Zinc-nickel Fokker 5-10 1100, 2000 10-12 >2000 

DERA 13.5 168 18.0 336 

ED Zinc-cobalt-iron Fokker 10 600 10-12 >2000 

DERA 16.8 216 17.7 1480 

PVD Aluminium DERA 19.9 96 22.8 168 

ED Aluminium DERA 15.1 96 15.3 230 

UBMS 

DERA 8.9 395 12.3 674 

Al-magnesium 

Delta-tone 


Delta-tone/Deltaseal 

Fokker 10-15 >1400 

DERA 16.9 1020 

(984) (984/985) 

Shorts >3723 >3723 

SermeTel Fokker 35 >2000 40 2000 

DERA 43.3 168 41.5 168 

Table C7; Neutral salt spray (ASTM B117) - panels scribed - Red rust in scratch 

NLR (Corrosion rating) 

Coating 500h 1000h 1500h 

ED Cadmium 3 4 5 

ED Cadmium passivated 1 1 1 

ED Zinc-nickel 4 5 5 

ED Zinc-nickel passivated 2/3 3 4 

ED Zinc-cobalt-iron 4 5 5 

ED Zinc-cobalt-iron passivated 3 4 4 

Sermetel 984 2 3 2 

Sermetel 984/985 1/2 2 2 

NLR (Corrosion rating 1-5) 1=slight (no) corrosion 5= severe corrosion 

Table C8; Summary of modified MASTMAASIS test results - 

panels scribed 

Page 66 




Coating Unpassivated Passivated 

Thick(μm) Time(h) Thick(μm) Time(h) 

ED Cadmium 15-20 1080 7 1080 

ED Zinc-nickel 5-10 >2000 10-12 >2000 

ED Zinc-cobalt-iron 10 >2000 10-12 912 

Delta-tone 


Delta-tone/Delta-seal 18.2 1080 

rust spots 

(984) (984/985) 

SermeTel 35 1080 40 1080 

Table C9; 100% humidity (ASTM D2247) - panels scribed (Time to rust in scribe) 

Exposure Site 

Coating 

DERA Schiphol NE Polder Warton 

Cadmium not passivated >536 >462 >365 

passivated >536 >462 >608 >365 

ED Zinc-nickel not passivated 225 91 252 

passivated 439 304 252 

ED Zinc-cobalt-iron not passivated >659 >462 >365 

passivated >659 >462 >608 >365 

IVD Aluminium not passivated 112 

passivated 184 

ED Aluminium not passivated 62 

passivated 62 

UMS Aluminium - not passivated 

magnesium 

Delta-tone >536 >462


DBAA DERA NLR NLR B Ae 

500mmol l -1 600mmol l -1 100mmol l -1 

NaCl pH 2 

100mmol l -1 

NaCl 

514mmol l -1 

NaCl 

Coating (SCE) (SCE) (Ag/AgCl) (Ag/AgCl) (Ag/AgCl) 

pass pass pass pass pass 

ED Cadmium -756 -747 -780 -775 -713 -710 -755 -754 -777 -745 

ED Zinc-nickel -1010 -996 -820 -1001 -765- 

912 

ED Zinc-cobalt-iron -977 -982 -1002 -980 -880 -941- 

1023 

PVD Aluminium -919 -750 -753 

ED Aluminium -881 -760 

UBMS Al-Mg -790 -885 

Delta-tone -1029 -1015 

-1118 -1005 -985 -1006 -1057 

-985 -1031 -1002 

SermeTel 984 -718 -703 -725 -570 -650 -616 -589 -706 -716 

Table C11; Open circuit potential measurements 

Average Ig d (μA.mm -2 ) 

Coating As-deposited Passivated 

ED Cadmium 0.61 1.66 

ED Zinc-nickel 0.97 1.50 

ED Zinc-cobalt-iron 0.78 1.28 

PVD Aluminium 0.37 0.74 


UBMS Al-Mg 1.52 1.05 

Delta-tone 1.05 

SermeTel 984 0.43 (984) 0.55(984/985) 

Table C12; Average galvanic currents determined from 

Scratch Model specimens in 600 mmol l -1 sodium chloride 

solution 

Page 68 



P d (mm) 


ED Cadmium 2 1.5 

ED Zinc-nickel 11 4 

ED Zinc-cobalt-iron 38 20.5 

PVD Aluminium 1.4 0.2 


UBMS Al-Mg 4.5 3.1 

Delta-tone 28 

SermeTel 984 0.2 (984) 0.3 (984/985) 

Table C13; Protection distance data for various metal coatings 



C.7 Figures 

Figure C1; Schematic diagram showing experimental arrangement 

used to conduct Scratch - Model experiments 

FigureC2; Schematic diagram showing experimental arrangement 

used to carry out cathodic protection measurements 

Page 70 



ANNEX D 

Galvanic compatibility 

D.1 Introduction 

An important feature of cadmium plating is its capacity to minimise the risk of galvanic 

corrosion occurring between steel fasteners and aerospace aluminium alloys. The 

electrochemical potential of cadmium plating is close to that for aluminium, so that the 

driving force for galvanic corrosion is small. Any cadmium replacements intended for use 

on fasteners must similarly reduce the risk of galvanic corrosion. 

In the present study two approaches have been employed to establish the galvanic 

compatibility of cadmium substitute coatings with aerospace aluminium alloys. The first 

approach has been to expose combinations of coated fasteners inserted into 2000 and 

7000 series aluminium alloy test blocks and coupons, to both accelerated corrosion tests 

and to natural exposure conditions. The second approach has been to determine the 

galvanic currents generated between different coatings and aluminium alloys using 

electrochemical measurements. 

D.2 Test procedures 

D.2.1 


Test specimens comprising of coated fasteners inserted into aluminium alloy blocks were 

exposed to continuous neutral salt fog. Two different designs of specimen were 

employed by Daimler Benz Airbus Aerospace and Fokker. Details of the arrangement 

used by Fokker are given in figure D1. After testing the specimens were disassembled 

and evidence of coating degradation and corrosion attack of the aluminium blocks was 

recorded. 

D.2.2 


Tests were carried out by Fokker at the Schiphol exposure site in The Netherlands. The 

specimen configuration was the one used in the neutral salt fog tests and illustrated in 

figure D2. The test blocks were removed after 10 months and examined for evidence of 

corrosion. 

D.2.3 

Galvanic corrosion current measurements 

The galvanic compatibility of the coatings with two aerospace aluminium alloys was 

investigated. The aluminium alloys selected for this study were an aluminium - copper 

alloy 2014-T6 and an aluminium - zinc - magnesium alloy, 7075-T6, both of which are 

used extensively in aircraft construction. Precise details of the galvanic coupling 

experiments have been given elsewhere [D1]. Figure D3 shows schematically the 

experimental arrangement employed. Briefly the test electrodes were held parallel and 

facing each other in a Perspex holder so that the gap between them was fixed at 25mm. 

They were then immersed in quiescent 600mmol l -1 sodium chloride solution (250ml) at 

25 o C. A zero-resistance ammeter was used to measure the current flow between the two 

electrodes. The experiments were run for 7 days. 

After the 7 day period, the aluminium alloy electrodes were removed from the sodium 

chloride solution and rinsed in distilled water. The aluminium alloy electrodes were then 

chemically cleaned in a hot chromic/phosphoric acid bath, rinsed in distilled water and 

acetone, and air-dried. After chemical cleaning, the weight change due to corrosion was 

calculated by subtracting the weight determined before coupling from the weight of the 

aluminium alloy after coupling and chemical cleaning. 



D.3 Results and discussion 

D.3.1 

D.3.1.1 


Daimler Benz Airbus Aerospace 

The results of neutral salt tests are summarised in table D1. The blocks were inspected 

on a weekly basis for evidence of rusting on the bolt heads. Data presented in table D1 

indicate that the four substitute coatings examined were less prone to rusting under 

these conditions than the electrodeposited cadmium control. The most promising coating 

was the Delta-tone/Delta coat which showed no evidence of rusting during the exposure 

period. 

D.3.1.2 

Fokker 

Metallographic examination of the aluminium alloy blocks after testing showed that 

corrosive attack had occurred in the countersink when cadmium plated bolts were used. 

No evidence of corrosion was found in the countersinks when there was contact with 

zinc-nickel and zinc-cobalt-iron plated bolts. 

D.3.2 

Natural exposure trials 

During the 10 month exposure period at the Schiphol test site no evidence of corrosion 

attack was observed. 

D.3.3 


In the first part of this study, the current flow between different coatings and two 

aerospace aluminium alloys was monitored. In all cases, some galvanic current flow was 

detected indicating that the coatings were interacting galvanically with the two alloys. 

Curves showing the variation in galvanic corrosion current with time for each of the 

coatings coupled to 2014-T6 and 7075-T6 aluminium alloys are reproduced in figures D4 

and D5 respectively. In most cases, the current was found to flow from the coating to the 

aluminium alloys. The exceptions were electrodeposited zinc-nickel alloy coating and 

Delta-tone where current reversal effects were seen after coupling periods of over 100 

hours. Tables D3 and D4 give the average current densities recorded for each of the 

coatings. In general, higher galvanic current values were observed for the more active 

zinc base coatings than the less active aluminium base coatings. 

The coupling of a metal coating with an aluminium alloy can cause corrosion of the 

aluminium alloy even if the coating is a sacrificial one. This occurs as a result of the build 

up of alkaline conditions on the aluminium alloy surface, which unlike in the case of steel 

which is stable at high pHs, can lead to damaging pit attack. In the second part of the 

work, therefore, the effect of galvanic coupling on the corrosion rate of the aluminium 

alloys was determined. 

The galvanic corrosion rate of the aluminium alloy, R g was calculated from the following 

expression, 

R g = R a - R o --------------------- (D1) 

where R o is the self corrosion rate and R a is the total corrosion rate of the aluminium 

alloy. 

The corrosion rates, R g, , of the two aluminium alloys were slightly increased by coupling 

with cadmium plating as indicated in tables D3 and D4. 

Page 72 



Since cadmium plating is usually regarded as being galvanically compatible with 

aluminium alloys a slight increase in the corrosion rate was considered acceptable. The 

alternative coatings were found to fall into two categories:- 

(1) Coatings that increased the corrosion rate of aluminium alloys above 

that found for cadmium plating 

(2) Coatings that lowered the corrosion rate of aluminium alloys below 

that found for cadmium plating 

The coatings, which fell into the first category, were ED Zinc-cobalt-iron, Delta-tone, PVD 

Aluminium, UBMS Aluminium - magnesium and SermeTel 984. The coatings that 

lowered the rate of aluminium alloy corrosion were ED Zinc-nickel and ED Aluminium. 

D.4 Conclusions 

1. Results of the neutral salt fog tests made on coated fasteners inserted into aluminium 

alloy blocks indicate that the Delta-tone coating is the most promising as no rusting 

was detected. 

2. Galvanic current measurements have shown that several of coatings, including ED 

zinc-cobalt-iron, Delta-tone, PVD aluminium, UBMS Al-magnesium and SermeTel 

984, increased the corrosion rate of the aluminium alloys above that found for 

cadmium plating. ED zinc-nickel and ED aluminium were found to lower the corrosion 

rate 

D.5 References 

[D1] 

Baldwin K.R. , Robinson M.J. and Smith C.J.E., “Galvanic corrosion behaviour of 

electrodeposited zinc and Zn-Ni coatings coupled with aluminium alloys” British 

Corrosion Journal 29 No.4 pp293-298 (1994) 



D.6 Tables 

Time hours 



ED Zinc-nickel 335 * 

ED Zinc-cobalt-iron 1000 

PVD Aluminium 1000 * 

ED Aluminium NT NT 

UMS Aluminium-magnesium NT NT 

Delta-tone/Delta-coat * * 

SermeTel (984) NT NT 

* No rusting detected NT - not tested 

Table D1; Kontakt Korrosions Test - Time to rust in screw heads 

Aluminium alloy Self corrosion rate, R o , 

(mg.dm -2 .day -1 ) 

2014-T6 5.9 

7075-T6 4.2 

Table D2; Self corrosion rates of aluminium alloys 

in 600mmol /litre sodium chloride solution 

Page 74 



Coating Passivated Mean Ig 

(µA.cm- 2 ) 

R a 

(mdd) 

R g 

(mdd) 

ED cadmium no 4.2 10.0 4.1 

yes 4.8 6.7 0.8 

ED zinc-nickel no 4.7 4.2 -1.7 

yes 9.2 1.2 -4.7 

ED zinc-cobalt-iron no 4.5 19.9 14.0 

yes 6.4 14.3 8.4 

PVD aluminium no 9.7 12.2 6.3 

yes 8.5 6.5 0.6 

ED aluminium no 9.5 9.1 3.2 

yes 9.1 4.4 -1.5 

UMS aluminium - 

magnesium 

no 6.1 14.2 8.3 

yes 4.9 9.9 4.0 

SermeTel 984 4.0 11.7 5.8 

984/985 4.7 6.7 0.8 

Delta-tone 9.6 16.5 10.6 

Table D3; Average galvanic current densities, R a and R g parameters for galvanic 

couples for 2014-T6 aluminium alloy in contact with metal coatings 

Coating Passivated Mean Ig 

(µA.cm- 2 ) 

R a 

(mdd) 

R g 

(mdd) 

ED cadmium no 4.4 7.1 2.9 

yes 4.2 6.2 2.0 

ED zinc-nickel no 5.0 3.3 -0.9 

yes 9.0 2.2 -2.0 

ED zinc-cobalt-iron no 5.1 14.4 10.2 

yes 7.6 10.6 6.4 

PVD aluminium no 8.6 12.0 7.8 

yes 7.0 10.4 6.2 

ED aluminium no 7.4 6.2 2.0 

yes 8.0 3.2 -1.0 

UMS aluminium - 

magnesium 

no 4.9 12.9 8.7 

yes 5.3 8.1 3.9 

SermeTel 984 4.1 9.3 5.1 

984/985 4.6 8.7 4.5 

Delta-tone 5.7 13.2 9.0 

Table D4; Average galvanic current densities, R a and R g parameters for galvanic 

couples for 7075-T6 aluminium alloy in contact with metal coatings 



D.7 Figures 

Figure D1; Design of block/bolt specimen used by Fokker Aircraft 

Figure D2; Diagram of apparatus used to monitor current flow 

between coated steel electrodes and aluminium 

alloy electrode 

Page 76 



Figure D3; Current density-time curves for various coatings coupled with aluminium 

2014-T6 alloy during immersion in quiescent 600mmol l -1 NaCl solutions 



Figure D4; Current density-time curves for various coatings coupled with aluminium 

7075-T6 alloy during immersion in quiescent 600mmol l -1 NaCl solutions 

Page 78 



ANNEX E 

Effects of coatings on fatigue performance 

E.1 Introduction 

Surface treatments such as etching, abrasive blasting, electroplating and anodising may 

have a serious effect on the fatigue resistance of a component. The aim of the testing 

programme undertaken was to establish the degree to which the cadmium substitute 

coatings reduce the fatigue strength of an aerospace grade, high strength steel. The 

testing was undertaken by SAAB, British Aerospace and the NLR. 

Detailed reports have been prepared by the NLR [E1] and SAAB [E2] and giving 

information about the test programme and fatigue data obtained. 

This annex outlines the test procedures used and summarises the main findings. 

E.2 Test procedures 

The influence of coatings on the fatigue crack initiation was determined using constant 

amplitude fatigue tests. Notched fatigue specimens were employed with stress 

concentration factors (K t ) of 1.4, 2.5 and 4. 

E.2.1 

Materials and specimen types 

Fatigue specimens used by NLR and SAAB were machined from 2.54cm diameter bars 

of AISI 4340 steel heat treated to HT200 strength level. Tensile tests performed by SAAB 

indicated that the strength level was 1400 MPa. Specimens used by BAe were prepared 

from BAC M53 steel, which was heat treated to the HT200 condition. 

The details of the specimen designs employed by the three laboratories are given in 

figure E1. 

E.2.2 

Coatings and surface pretreatments 

The fatigue test programme covered the following coatings:- 

• ED cadmium, ED zinc-nickel, ED zinc-cobalt-iron, 

• SermeTel, Delta-tone, 

• UBMS aluminium - magnesium, ED aluminium 

In addition tests were conducted on samples which had not been plated. 

The normal treatment prior to applying the coating was to stress relieve the specimens at 

420 o C for 1 hour and then to alumina grit blast the surface. Further details are given in 

Annex A and in reference E1. 

E.2.3 

Fatigue testing 

The fatigue testing was carried out using High Frequency Pulsator (HFP) machines. The 

test frequency was in the range 150 to 200 Hz and the applied stress ratio R=0.1. 

At SAAB and BAe testing was continued until failure occurred or the total number of 

cycles exceeded 10 7 . In the SAAB test programme specimens, which had not failed after 

10 7 cycles were generally re-tested at a higher stress level. At NLR fatigue testing was 

continued until 5 to 10 x10 7 cycles was reached. 



E.3 Results and discussion 

S-N curves were obtained for each of the coatings evaluated. Figure E2 gives examples 

of curves obtained for specimens electroplated with zinc-cobalt-iron and zinc-nickel. In 

these tests the specimens had a stress concentration factor (K t ) of 1.4. 

All the S-N curves obtained are given in reference E1 together with the individual fatigue 

specimen results. 

Data obtained from the S-N curves were used to calculate the relative effects of the 

different coatings on the fatigue strength of the steel. These are summarised in table E1. 

The fatigue strength for each of the coatings is compared with the fatigue strength for 

specimens, which have been abrasive blasted with alumina grit. 

The results obtained show that the coating causing the greatest reduction in fatigue 

strength is ED zinc - nickel. ED zinc-cobalt-iron was similar to ED cadmium giving a 

reduction of ~10% whilst the other coatings investigated resulted in a lowering in fatigue 

strength of about 5%. 

Several factors will influence the initiation of fatigue cracks on coated specimens. These 

include, 

• coating thickness 

• surface roughness due to grit blasting 

• peening effect due to grit blasting 

• strength of the base material 

• hydrogen embrittlement 

A detailed study has been made at NLR [E1] in order to identify the impact of these 

factors on the fatigue life. Metallographic sectioning has shown that there are 

considerable variations in the thickness of the coating within the notched region for each 

of the coatings. There were also found to be large differences in the surface roughness 

as a result of the pre-coating abrasive blasting treatment. This arises from the fact that 

the abrasive blasting was carried out by each of the organisations responsible for the 

different coating processes. Although surface roughness will influence the fatigue crack 

initiation, NLR's study suggests that the embedded aluminium oxide particles may act as 

preferred crack initiation sites. 

E.4 Conclusions 

1. Based on constant amplitude tests the following conclusions can be drawn 

concerning the effects of coatings on the fatigue strength on steel. 

2. By the aluminium oxide blasting treatment before the coating process the surface is 

roughened but also aluminium oxide particles are trapped in the surface metal. 

3. Entrapped oxide particles are potential fatigue crack initiation sites. 

4. The ED Zinc - nickel coating resulted in the largest reduction in fatigue strength, 

25%. This was due to the hard brittle coating and the relatively large coating 

thickness. 

5. ED zinc - cobalt - iron gave no larger reduction in fatigue strength than cadmium, 

~10%. The negative effect of cadmium on fatigue strength is unexplained. 

Page 80 



6. For coatings ED aluminium, SermeTel 984, Delta-tone and UBMS aluminium - 

magnesium only a slight detrimental effect ~5% on the fatigue strength was found. 

E.5 References 

[E1] 

[E2] 

‘t Hart W.G.J. and Boogers J.A.M., Influence on fatgiue crack initiation of 

environmentally friendly coatings to replace Cadmium, NLR-CR-98014 

(1998) 

Magnusson L., Fatigue test results of SAAB work in GarteurAG17, Cadmium 

substitution in Aircraft, TUDL-R-4073 (1996) 



E.6 Tables 

Laboratory Coating K t = 1.4 K t = 2.5 K t = 4 

Bare (blasted) 1.0 1.0 1.0 

ED Cadmium 0.85 0.90 

ED Zinc - nickel 0.80 0.75 

NLR ED Zinc - cobalt - iron 0.90 0.90 


SermeTel 984 0.95 0.95 

Delta - tone 0.90 0.95 

UBMS - Al - magnesium 

SAAB 

ED Cadmium 0.85 0.90 0.90 

ED Zinc - nickel 0.75 0.75 

ED Zinc - cobalt - iron 0.90 0.90 1.00 

ED Aluminium 

SermeTel 984 0.90 0.90 

Delta - tone 0.95 0.90 

UBMS - Al - magnesium 1.00 0.80 

ED Cadmium 0.85 

ED Zinc - nickel 

British ED Zinc - cobalt - iron 0.90 

Aerospace ED Aluminium 

SermeTel 984 0.90 

Delta - tone 

UBMS - Al - magnesium 0.95 

Table E1; Effect of coating on fatigue strength - performance relative to bare steel 

Page 82 



E.7 Figures 

Figure E1; Design of fatigue specimens 



Figure E2; S-N curves for specimens coated with ED zinc-nickel and ED zinc-cobalt-iron 

N= tested by NLR, S=tested by SAAB 

Page 84 



ANNEX F 

Stress corrosion cracking studies 

F.1 Introduction 

High strength steels are susceptible to hydrogen embrittlement and stress corrosion 

cracking. Conventional aqueous electrodeposition processes are less than 100% 

efficient and during electroplating some hydrogen may diffuse into the steel component. 

This may lead to failure of the component under tensile loads as a result of hydrogen 

embrittlement. In order to minimise the risk of failure, components are baked after 

plating, typically at temperatures of between 190 and 230 o C. The duration of the deembrittlement 

treatmen is dependent on the ultimate tensile strength of the steel. For 

steels with maximum tensile strengths in the range 1101 MPa to 1450 MPa the 

European Standard EN 2133 advises a minimum baking time of 8 hours after cadmium 

plating [F1]. Steels in the strength range 1451 to 1800 MPa require longer baking 

treatments usually at least 18 hours whilst for ultra high strength steels, i.e. above 1801 

MPa, times in excess of 24 hours must be employed [F2]. 

Under certain conditions, parts manufactured from high strength steels may fail by stress 

corrosion cracking. For failure to occur the component must be under tensile stress and 

be exposed to a corrosive environment. The mechanism of stress corrosion cracking in 

high strength steels is normally hydrogen embrittlement. Hydrogen may be introduced 

into steels as a result of corrosion processes occurring on the surface of the component. 

A damaged coating, for example, may lead to the establishment of a galvanic corrosion 

cell between the coating and steel substrate. Hydrogen generated as a result of the 

cathodic reaction may diffuse into the steel, ultimately leading to hydrogen embrittlement 

failure. 

In the current programme both aspects of hydrogen embrittlement have been 

investigated. 

F.2 Test procedures 

F.2.1 

Hydrogen embrittlement 

Sustained load testing and slow bend tests were carried on a limited number of coated 

and heat treated samples to establish whether the recommended de-embrittlement 

treatments were effective [F3]. 

Notched tensile specimens were manufactured from AISI 4340 steel heat treated to 1790 

- 1930 MPa. The specimens were then electroplated with zinc-nickel and subsequently 

de-embrittled by heat treating at 190 o C for 3 hours. Two series of 3 test pieces were 

placed under sustained load for 200 hours at 75% of the notched tensile strength. The 

tests were conducted at room temperature. 

A limited number of slow bend tests were conducted by Aerospatiale on a 35NCD16 

steel heat treated to 1800MPa. The tests were conducted in accordance with prEN 2831. 

The initial failure angle, α 0 , and the failure angle after surface treatment, α 1 , were 

measured. Details are given in reference [F4]. 

F.2.2 

Stress corrosion cracking 

The risk of stress corrosion cracking occurring as a result of corrosion has been studied 

using notched tensile specimens. These were loaded to 75% of the notched tensile 

strength after coating, and are then subjected to alternate immersion in 3.5% sodium 

chloride solution until failure occurs. The performance of each of the coatings has been 



compared with cadmium plating. Specimens with a configuration shown in figure F1 were 

stress corrosion tested to ASTM G44-75. In this test, the specimens were subjected to 

alternate immersion in 3.5% sodium chloride solution. One cycle in this standard 

includes 10 minutes in salt solution and fifty minutes drying in air. The total exposure 

time was 30 days. 

The notched tensile strength was determined on three specimens. Values of 2700, 2705 

and 2690 M Pa were achieved. For one set of stress corrosion specimens the notch was 

covered to 180 o by an aluminium casing. The reason for using this casing is to initiate 

galvanic corrosion and crevice corrosion. The specimens were then loaded to and held 

at 75% of the notched tensile strength. Each treatment was represented by three 

individual specimens The treatments tested were:- 

F.3 Results and discussion 

• Reference without protective coating 

• Porous cadmium plating 

• Zinc-nickel plating 

• Zinc-cobalt-iron plating 

• Sermetel 

• Electrodeposited aluminium 

• Delta-tone 

F.3.1 

Susceptibility to hydrogen embrittlement 

The results of slow bend tests conducted on a 35 NCD 16 steel (1800 MPa) are 

presented in figure F1. The results indicate that for the SermeTel coatings, there is 

minimal embrittlement due to plating. In the case of the electrodeposited aluminium 

some reduction is observed but this may be recovered by heat treating at 190 o C. 

One of the three electroplated zinc-nickel notched tensile specimens tested, failed within 

the 200 hours test period. In the second series of specimens tested, no failures occurred 

and it is considered that the electrodeposited zinc-nickel coating is non-embrittling. 

F.3.2 

Susceptibility to stress corrosion cracking 

In the sustained load tests, two specimens of three of those protected with zinc-cobaltiron 

and those protected with Delta-tone coating in the test set using the aluminium 

casings, failed within two days. Sacrificial coatings may add to the risk of post plating 

embrittlement. SAAB uses ZnCoFe plating to 1250 M Pa, a strength level considered to 

be safe. The specimens used in this investigation were hardened to 1400 M Pa. 

In the second test set, where no casing was used , two specimens of three with Deltatone 

coating failed within two days. All other specimens remained unbroken after 30 

days. 

Previous research undertaken by SAAB [F2] compared the performance of IVD 

aluminium and electrodeposited cadmum coatings applied to notched tensile specimens 

machined from either a 1366 Ni-Cr-Mo steel or a 1745 precipitation hardening stainless 

steel. As in the current research programme, the specimens were loaded to 75% of the 

notched tensile strength. From the tests undertaken, there was no indication that 

aluminium-coated specimens have a higher tendency to delayed fracture in a corrosive 

environment than cadmium coated ones. 

F.4 Conclusions 

Page 86 



1. The Delta-tone and electrodeposited zinc-cobalt-iron coatings were found 

to promote susceptibility to stress corrosion cracking when exposed to 3.5% sodium 

chloride solution 

2. Additional testing, including the slow bend test, indicates that 

susceptibility to hydrogen embrittlement may be minimised by post plating heat 

treatments. 

F.5 References 

[F1] 

Hultgren E., CSM Materialteknik Technical report T60676MM.991 

“Stress corrosion testing of AISI 4340 coated with cadmium, zinc alloys and 

aluminium.” (1996) 

[F2] Jaensson B., SAAB-SCANIA Investigation Report KFB-033 

“Replacement of cadmium as a corrosion" 

[F3] 

(1996) 

Marchandise D, Hydrogen embrittlement data - Aerospatiale report 

[F4] 

PrEN 2831 Aerospace Series Hydrogen embrittlement of steels. Test by 

slow bending (1990) 



F.6 Figures 

Figure F1; Summary of slow bend test data for electrodeposited 

aluminium and SermeTel coatings 

Page 88 



ANNEX G 

Resistance to aircraft chemicals 

G.1 Introduction 

During an aircraft's operational life it comes into contact with a large number of 

chemicals and fluids. These include aviation fuel, engine oils and hydraulic fluids as well 

as cleaners and paint strippers used in the maintenance and repair of protective 

treatments. The aim of the work described below was to establish whether any of the 

cadmium replacements being investigated were likely to degrade on prolonged contact 

with some of the commonly used aircraft chemicals. 

G.2 Test procedures 

Studies of the resistance of replacement coatings to aircraft fluids were carried out by 

British Aerospace and Shorts. Details of the fluids examined by each of the organisations 

are given in table G1. 

G.2.1 


All specimens were subjected to immersion in the above fluids for a period of 7 days. 

Following immersion the panels except for the water and Propanol were wiped clean with 

a clean cloth moistened with Petroleum spirit and left to dry. The remaining panels were 

wiped with a dry clean cloth. On completion of above the panels were visually assessed 

using x10 magnification before carrying out microscopic evaluation using an optical 

microscope. 

G.2.2 

Shorts 

G.3 Results 

Coated panels 100 x 150mm were immersed in the various fluids identified in table G1. 

The panels were inspected daily for signs of corrosion attack, coating removal and colour 

changes. 

Tables G2 and G3 summarise the results obtained from the B.Ae and Shorts studies. 

Several of the coatings investigated, including electrodeposited cadmium, were found to 

be susceptible to corrosion attack when in contact with the aircraft cleaner Turco 5948. 

For ED Zn/Ni (passivated and non-passivated) the cleaner starts to remove the coating 

within 24 hours whilst for ED Cadmium (passivated and non-passivated) some attack is 

apparent after 500h immersion. 

Data presented in table G3 further indicate that Skydrol has a detrimental on both 

electrodeposited cadmium and zinc-cobalt-iron coatings. In each case a significant 

weight loss was recorded. 

G.4 Conclusions 

Most of the replacement coatings examined failed to show any significant degradation on 

exposure to a range of chemicals used on aircraft. Exceptions were ED zinc-nickel 

coatings in contact with Turco 5948 and ED zinc-cobalt coatings immersed in Skydrol 

hydraulic fluid. Cadmium plating was also found to be degraded by these fluids. 

G.5 Tables 




Shorts 

Hydraulic fluids (OM15) Skydrol 

Aviation fuel Avtur F34 Jet A1 

Engine Oil DERD 2497 

Cleaners 

Water(40 o C) 

Propan-2-ol 

Turco DPM 

Ethylene glycol 

Table G1 ; Aircraft fluids examined 

Test fluids 

Coating 

post plating 

treatment 

Propan-2-ol 

Hydraulic 

oil (OM15) 

Fuel 

(Avtur) 

Engine Oil 

(DERD 

2497) 

Water 

(40 o C) 

ED Zinc - Ni none p p p p f 

passivated p p p p p 

ED Zn-Co-Fe none p p p p f 


ED Cadmium none p p p p p 


SermeTel none p p p p p 

topcoat p p p p p 

p - no evidence of deterioration 

f - moisture capable of attacking the grain boundaries of the coating allowing water to 

penetrate and get under the coating layer to produce a flaking effect if retained in this 

condition for any greater length of time. 

Table G2; Coating performance determined using B.Ae test procedure 

Page 90 



Test fluids 

Coating 

post plating 

treatment 

Ethylene 

Glycol 

Turco 5948 Jet A1 Skydrol 

ED Zinc - Ni none p general attack 

(-93mg) 

passivated p edge attack 

(-125mg) 

p 

p 

p 

p 

ED Zn-Co-Fe none p edge attack 

(-61mg) 

passivated p slight edge attack 

(-15 mg) 

p 

p 

discolouration 

(-19mg) 

p 

(+30 mg) 

ED Cadmium none p 

(-11mg) 

general attack 

(-140mg) 

passivated p general attack 

(-334mg) 

p 

p 

discolouration 

(-22mg) 

p 

(-68mg) 

SermeTel none p p p p 

topcoat p p p p 

Delta-tone 

p 

(+18mg) 

p 

(+166mg) 

p 

p 

(+14mg) 

ED Aluminium p p p p 

IVD Aluminium none p 

(+23mg) 

passivated p 

(+17mg) 

slight edge attack 

(+19mg) 

slight edge attack 

(+15mg) 

p - no visual damage 

Table G3; Coating performance determined using Shorts total immersion tests 




Page 92 



ANNEX H 

Paint adhesion 

H.1 Introduction 

The adhesion of paint on the different coatings was determined by performing tensile 

tests on tapered painted strips and using cross-cut tests. Table H1 summarises the 

coatings and tests employed. 

H.2 Test procedures 

H.2.1 

Cross-cut test 

Two versions of the cross-cut test were employed. 

The cross cut test carried out by Aerospatiale [H1] used a regular square grid pattern. 

Tests were conducted on as painted panels and on painted panels, which had been 

immersed in de-mineralised water for 14 days. The degree of paint adhesion was 

determined by comparing the appearance of the grid after testing with standards given in 

ISO 2409-72. Polyurethane Paint System - PAC33 + PU66 

The adhesion quality for the paint system on the steel substrate was checked by NLR 

using the cross cut test according to NEN 5337 [H2]. A square grid of scribe lines at 

increasing distances from 01 to 1.1mm was applied with a sharp chisel on the painted 

surface, after which the loose paint blisters were removed with Scotch tape. The degree 

of paint adhesion can be expressed in mm delamination. The cross cut tests were 

performed on unexposed paint systems and after 7 days immersion in distilled water. 

H.2.2 

Low temperature tensile test 

H.3 Results 

The tensile specimen is tapered in the length direction to obtain a gradual plastic strain 

distribution along the specimen axis as a result of tensile testing. After testing 

macroscopic photographs were made to show the influence of plastic deformation of the 

substrate on paint delamination. Cross sections gave information on crack density at 

increasing plastic strain. Reference H2 gives details of the dimensions of the tensile 

specimens used and the approximate plastic strain distribution after tensile testing. 

H.3.1 

Cross-cut tests (regular square grid) 

Results obtained by Aerospatiale using a regular square grid are presented in table H2. 

Adhesion to the electrodeposited aluminium coating was found to be very good. Some 

slight loss in adhesion was reported when panels were immersed in distilled water for 14 

days. In the case of the Sermetel 984 LT coating, paint adhesion was poor when 

samples were immersed in distilled water prior to testing. 

H.3.2 

Cross-cut tests (variable grid size) 

Table H3 summarises the results of the cross-cut tests performed on coated panels that 

have been painted. In some instances (coatings 1,3 and 5) only moderate paint adhesion 

was recorded. This may be associated with the fact that the paint scheme was not 

applied within the specified time following passivation. Excellent paint adhesion was 

achieved with coatings 2 and 5 where application of the paint scheme had been carried 

out within the required period. The paint adhesion of the Sermetel after one week 



immersion was poor although the coating supplier claims that no additional paint system 

is necessary. 

H.3.3 

Tensile tests on tapered specimens 

Paint adhesion was determined by low-temperature (-50 o C) testing of coated and painted 

specimens. As compared to the Sermetel adhesion, the paint adhesion on the metallic 

coatings was poor. Complete paint delamination occurred for deformations larger than 

3%. 

H.4 Conclusions 

The cross-cut tests show that good paint adhesion may be achieved with metal coatings. 

An important factor is the time delay between passivation and the application of a primer. 

Data obtained by NLR indicate that if the passivated surfaces are exposed to the 

atmosphere for too long, poor paint adhesion may be obtained. 

H.5 References 

[H1] 

t’Hart W.G.J. and Boogers J.A.M., Coatings considered for cadmium 

substitution Garteur AG17 Programme, NLR-CR-97107 L 

[H2] Marchandise, D., Synthesis of Aerospatiale Results, (1996) 

Page 94 



H.6 Tables 

Coating 

Cross Hatch Test 

(1) Aerospatiale 

Cross Hatch Test 

NLR (1) 

Tensile test on 

tapered specimens 

NLR 


ED zinc-nickel 3 3 

ED zinc-cobalt 3 3 

ED aluminium 3 

SermeTel 984 3 3 3 

Table H1; Summary of paint adhesion tests carried out 

Coating 

Initial 

14 days 

immersion in 

demin. Water 

1 1 2 2 

1 2 

ED Aluminium 1 1 2 2 

1 2 

1 1 2 2 

1 2 

1 2 5 5 

3 5 

SermeTel 2 2 5 5 

CR 984 LT 2 5 

2 2 5 5 

2 5 

Table H2; 



Coating Dry After 

immersion 

1 ED cadmium passivated painted 0.3 0.3(0.5) 

2 ED cadmium vapour degrease + painted 0.1 0.1 

passivation 

3 ED zinc-cobalt iron passivated painted 0.4(0.8) 1.1 

4 ED zinc-nickel passivated painted 0.2 0.8(1.1) 

5 ED zinc-nickel vapour degrease + painted 0.1 0.1 

passivation 

6 Sermetel 984/985 solvent wipe painted 0.2 1.1 

7 Sermetel 984/985 light abrasion + painted 0.2 1.1 

solvent wipe 

Table H3; Results of cross cut tests 

Page 96 



ANNEX I 

Tribological properties 

I.1 Introduction 

The tribological properties of the coatings are an important consideration when used on 

fasteners or on threaded parts. In the tightening up of a bolt most of the effort expended 

is used in overcoming the frictional constraints. Cadmium has a low coefficient of friction 

(~0.2) so that a comparatively low torque need be applied to achieve a given level of preload. 

During the assembly of aircraft structures it is often necessary to bolt parts into 

place and then remove them to make adjustments or modifications. This process may be 

repeated several times and it is important that the torque-tension characteristics of the 

coated bolts used remain unaltered. In order to determine the suitability of the different 

coatings for use on fasteners the current programme has determined the coefficient of 

friction of several of the coatings and has compared the torque-tension characteristics of 

coated fasteners after repeated tightening and untightening. 

I.2 Torque - tension measurements 

I.2.1 

Hi-Lok fasteners 

Two systems were evaluated:- 

(1) steel pins and steel collars 

(2) titanium pins and steel collars 

For each system the preload obtained and the locking torque was recorded. Tables I1 

and I2 summarise the data for tests made on steel and titanium pins respectively. The 

figures given represent the average of 7 measurements. The results obtained indicate 

that the level of preload and locking torque values are dependent on the coating system. 

The normal requirement is that the preload should be in the range 4 to 9kN. As indicated 

in tables I1 and I2 the minimum preload of 4kN was exceeded with all of the coatings 

examined. 

I.2.2 

Coated steel bolts 

Results of the torque - tension tests conducted by SAAB on steel bolts and nuts coated 

with cadmium and various alternative coatings are given in table I3. Data are also 

included in the table previously obtained at SAAB [I1] and at DERA [I2,I3]. 

I.3 Reusability tests 

Tests were made to determine the effect of repeated tightening up and untightening on 

the performance of coated nuts and screws. The work undertaken by DBAA has 

concentrated on fasteners coated with cadmium and Delta-tone. Data obtained are 

presented in figure I1 showing the effect of repeated loading and unloading on the 

coefficient of friction (μ). Different combinations of cadmium and Delta-tone plated steel 

bolts and nuts have been examined. The results indicate that when the bolt and nut are 

both cadmium plated there is an increase in the coefficient of friction as the combination 

is repeatedly tightened and untightened. When either the nut or bolt is coated with Deltatone 

there is only a small change in the value of μ. Data are presented in figure I2 

showing the effects of repeated loading on the preload to torque-off ratio. This shows 

that the largest decrease occurs with the cadmium plated bolt-cadmium plated nut 

combination. 



Additional data obtained by DERA in earlier research [I3] is given in figure I3 for PVD 

aluminium and PVD aluminium - 5% magnesium coated bolts and nuts. The data points 

represent the torque values required to achieve each incremental bolt load. For each 

coating type, data are presented for lubricated and dry fasteners recorded on the first 

and fifteen nut runs. The scatter in results indicates that the bolt to bolt variation in 

torque-tension characteristics, are greatest for non-lubricated fasteners. Repeated 

installations upto 15 nut runs resulted in the degradation of the torque-tension 

relationship for the PVD coatings. In some instances seizure of the fasteners occurred 

and often very high torque levels were required to achieve a modest 2kN preload. 

I.4 Coefficient of friction 

Values for the coefficients of friction for the various coatings examined in this programme 

are compared in table I4. Data obtained from earlier work carried out at DERA [I2,I3] is 

included together with some published values. With the exception of the Delta-tone 

coating, the coefficient of friction measured is higher than for cadmium plating. The value 

of the coefficient of friction is sensitive to the measurement method employed. This is 

reflected in the range of values obtained for electroplated cadmium. 

I.5 Conclusions 

1. For a given locking torque, the preload level recorded is dependent on the coating 

system. 

2. All the coatings examined resulted in pre-loads on Hi-Lok fasteners greater than the 

minimum 4kN required. With the ED cadmium, ED aluminium, ED Zn-Co-Fe and ED 

Zn-Ni coated steel fasteners the maximum preload of 10kN was exceeded. 

3. With the exception of the Delta-tone coating, coefficients of friction greater than that 

found for cadmium plating have been measured. 

4. Torque-tension tests conducted on combinations of cadmium and Delta-tone coated 

bolts and nuts have shown that with repeated loading, the greatest change in 

properties occurs when both the nut and bolt are cadmium plated. The change in 

coefficient of friction and preload is small if one of the elements is plated with Deltatone. 

5. Previous work indicates that PVD aluminium coatings may be ineffective on bolts 

subjected to repeated loading. 

I.6 References 

[I1] 

[I2] 

L Magnusson, "Electroplated coatings of Zn-Co-Fe and Zn-Ni. Effect on 

fatigue properties and tension-torque", SAAB report TUDL R-4015 (1993) 

V C R McLoughlin, "An evaluation of coatings for steel and titanium alloy 

fasteners for aircraft applications", AGARD Conference Proceedings No.256, 

Advanced Fabrication Processes Florence, Italy (1978) 

[I3] 

P L Lane and C J E Smith, “Development of aluminium-alloy coatings for 

the protection of steel”, published in Surface Engineering Practise (ed. K N 

Strafford, P K Datta and J S Gray) pp566-577 Ellis Horwood Ltd. Chichester (1990) 

Page 98 



[I4] 

C A Boose and R J A Blankers, "Investigations of the properties of 

electrodeposited SIGAL® galvano aluminium coatings", TNO Report 85M/016089 

BLA/AKU (1984) 



I.7 Tables 

Coating Preload (kN) Locking torque 

ED cadmium 10.78 0.69 

Delta-tone 5.20 1.54 

PVD (Al-Mg) 7.75 1.07 

ED aluminium 9.32 0.71 

ED Zn Co Fe 10.95 0.79 

ED Zn Ni 9.25 0.84 

Table I1; Preload and locking torque values for coated 

steel pins with coated steel collars 

Coating Preload (kN) Locking torque 

ED cadmium 8.31 1.28 

Delta-tone 5.88 1.01 

PVD (Al-Mg) 6.44 1.51 

ED aluminium 8.63 0.97 

ED Zn Co Fe 10.42 0.70 

ED Zn Ni 9.60 0.69 

Table I2; Preload and locking torque values for coated 

titanium pins with coated steel collars 

Load (kN) 

Coating Torque 7 Nm Torque 17.5 Nm 

SAAB 

(a) (c) (d) 

SAAB 

(b) (e) 

ED Cadmium 7.3 5.37 10.96 14.0 9.32 

PVD aluminium 4.2 3.69 9.00 8.15 

ED aluminium 5.3 

ED zinc-cobalt-iron 10.5 

ED zinc-nickel 8.0 8.90 

Delta-tone 

PVD aluminium-5%magnesium 8.7 

(a) Early SAAB data ref. (I1) 

(b) Garteur coatings 

(c) Early DERA data ref.(I2) (d) Early DERA data ref.(I3 ) 

(e) Ref. (I4) 

Table I3; Tension loads for coated bolts installed with lubricant 

Page 100 



Coefficient of friction 

Coating DBAA DERA (1) (2) 

ED cadmium 0.129 0.21 0.54 

PVD aluminium + chromate 0.41 

ED aluminium + chromate 1.14 

ED zinc-cobalt-iron 

ED zinc-nickel 0.66 

Delta-tone 0.106 

PVD aluminium-5%magnesium + chromate 0.35 

ED zinc + chromate 0.76 

Table I4; Coefficient of friction 



I.8 Figures 

Cadmium plated bolt / cadmium plated nut 

Cadmium plated bolt / Delta-tone coated nut 

Delta-tone coated bolt / Cadmium plated nut Delta-tone coated bolt / Delta-tone coated nut 

Figure I1; Effect of repeated tightening and untightening on the coefficient of friction for 

various combinations of cadmium and Delta-tone coated bolts and nuts 

Page 102 



Figure I2; Effect of repeated tightening and un tightening on the preload to torque-off 

ratio for various combinations of cadmium and Delta-tone coated bolts and 

nuts 



Cadmium coated bolt and nut 

1 st installation 15 th installation 

PVD aluminium coated bolt and nut 


PVD aluminium - 5% magnesium coated bolt and nut 


Figure I3; Torque-tension relationships for steel fasteners coated with cadmium, 

aluminium or aluminium - 5% magnesium on the 1 st and 15 th installations 

Page 104 



ANNEX J 

Coating repair 

J.1 Introduction 

The use of brush plating for the repair of electrodeposited zinc-nickel, zinc-cobalt-iron 

and PVD aluminium coatings was investigated. The approach adopted was as follows: 

J.2 Experimental 

• Establish the corrosion resistance of brush plated coatings in comparison with 

bath applied coatings 

• Examine the corrosion resistance of coated panels which had been damaged 

and then repaired by brush plating 

J.2.1 

Brush plating of mild steel panels 

A series of trials were conducted initially using commercially available cadmium and 

alkaline zinc-nickel brush plating electrolytes and experimental acid zinc-cobalt and acid 

zinc-nickel electrolytes. Mild steel 1mm thick 50mm x 50mm panels were brush plated to 

produce coatings of various thicknesses in the range 5 to 25μm. Tape pull-off tests were 

performed on several of the coated samples to check coating adhesion. Neutral salt fog 

trials were also conducted and the time to red rust recorded. 

J.2.2 

Brush plating of damaged panels 

J.3 Results 

Coated panels prepared for the Garteur programme were first damaged by removing a 

portion of the deposited layer in the centre of the test panel. They were then re-plated in 

the damaged region itself and around the edge of the original coating with either zincnickel, 

zinc-cobalt alloy or cadmium coatings (see figure J1) according to the matrix in 

table J1. 

The adhesion of electrodeposited coatings to aluminium substrates is frequently poor 

because of the presence of the surface oxide film. Several samples of the panels coated 

with PVD aluminium were treated prior to re-plating with cotton swabs containing zincate 

solution. The zincate process employed was a relatively simple, cheap and reliable 

technique that has often found use when there is a requirement to plate onto aluminium. 

The corrosion resistance of the re-plated panels was established using neutral salt spray 

tests. 

J.3.1 

Brush plated mild steel panels 

Each of the electrolytes evaluated was found to provide suitable coatings in the required 

thickness ranges. The zinc-nickel and cadmium coatings were found to be semi-bright at 

best, whereas the zinc-cobalt coatings were bright in appearance. In the tape pull-off 

tests no signs of adhesion failure were observed. 

The results of the neutral salt fog tests made on coatings nominally 7-8μm thick are 

summarised in table J2. 

The data indicate that the brush plated zinc-cobalt coating provides the highest level of 

corrosion protection. The two zinc-nickel coatings examined were less effective than 



cadmium although it can be seen that as the nickel content was increased so the time to 

red rust was extended. 

J.3.2 

Neutral salt fog tests on repaired panels 

The re-plated panels as described by the matrix in table J1 were subjected to neutral salt 

fog for a period of 336 hours and then inspected for signs of corrosion. It was found that 

the Garteur cadmium coatings were satisfactorily re-plated with cadmium, zinc-nickel or 

zinc-cobalt alloy. Adhesion tests carried out on re-plated coatings showed no signs of 

failure. On exposure to neutral salt fog, the repair coatings of zinc-nickel and zinc-cobalt 

failed within 336 hours, as anticipated from the data shown in table J2, where in each 

case red-rust occurred after 110 and 310 hours respectively. However no red-rust was 

observed on the Garteur coatings. 

When re-plating a metallic layer with a coating of a dissimilar metal there is always the 

risk that galvanic corrosion may occur at the interface between the two coatings. In the 

present study, no evidence of galvanic corrosion was observed where the coatings 

overlapped for the cadmium - cadmium or cadmium - zinc-nickel combinations, indicating 

that they were fully compatible. In contrast, when the zinc-cobalt alloy was employed in 

re-plating, the cadmium coating was badly pitted around the interface, suggesting that 

galvanic corrosion had occurred leading to attack on the cadmium plating. For cadmium, 

which had been repaired with zinc-cobalt, it was found that after 380 hours, red rust 

started to bleed from the interface showing that the patch had broken down. Table J3 

summarises the occurrence of galvanic corrosion between the original coatings and the 

repair coatings. 

The repair of PVD aluminium coatings presented particular difficulties due to adhesion 

problems and the zincate treatment was applied in each case. For cadmium, it was 

found that the coatings could not be brush plated onto aluminium despite the use of the 

zincate treatment. However it was found that the zincate treatment could be successfully 

employed in the brush plating of alkaline zinc-nickel coatings onto PVD aluminium. In the 

absence of the zincate treatment adhesion failures occurred. In contrast, for the acidic 

zinc-nickel and zinc-cobalt electrolytes no adhesion problems were observed even in the 

absence of the zincate treatment. In the neutral salt fog tests the zinc-nickel and zinccobalt 

coatings were found to afford good protection with no red rust on the re-plated 

areas. In the absence of the zincate treatment, the more electrochemically active zinccobalt 

coating caused some sever galvanic corrosion on the aluminium coating, 

however, when the zincate treatment was applied this effect was considerably reduced. 

J.4 Conclusions 

1. In neutral salt fog trials, the zinc-cobalt alloy was found to provide the highest levels 

of corrosion protection to a mild steel substrate giving greater protection than either 

cadmium or zinc-nickel alloy plating. 

2. Simulated corrosion damage and re-plating tests showed that brush plated zinccobalt 

and zinc-nickel coatings could be used to re-protect a range of Garteur 

coatings including zinc-nickel, zinc-cobalt-iron, PVD aluminium and cadmium. 

3. Some evidence of galvanic interactions were observed when using the brush plated 

zinc-cobalt alloy coating for repair. These effects were minimised when a zincate 

chemical treatment was used. 

J.5 Tables 

Page 106 



Garteur coatings Cadmium Acid 

Zn -Co 

Brush - plated coatings applied 

(electrolyte used shown in brackets) 

Acid 

Zn-Ni 

Alkaline Zn- 

Ni 

ED cadmium 4 4 4 

ED Zn-Co-Fe 4 4 

ED Zn-Ni 4 4 

IVD Aluminium 4 4 4 4 

Table J1; Brush-plated coatings applied to Garteur coatings 

Electrolyte Alloy composition (wt%) Thickness Time to red 

Zn Ni Co Cd μm rust (hrs) 

Acid zinc-nickel 89.5 10.5 7.5 158 

Alkaline zinc-nickel 92.2 7.8 8.0 110 

Acid zinc-cobalt 99.0 1.0 7.5 310 

Cadmium 100 8.1 148 

Table J2; Corrosion data for 8μm brush -plated coated steel 

exposed to neutral salt fog 



Garteur main coating Repair coating Zincate 

treated 

Adhesion of 

repair coating 

Galvanic 

corrosion 


Pass 

Cadmium Alkaline zinc-nickel Pass 

Acid zinc-cobalt Pass Observed 

Zinc-nickel Alkaline zinc-nickel Pass 

Acid zinc-cobalt 

Pass 

Zinc-cobalt-iron Alkaline zinc-nickel Pass 

Acid zinc-cobalt 

Pass 


Fail 

Cadmium 4 Fail 

Alkaline zinc-nickel 

Fail 

Alkaline zinc-nickel 4 Pass 

PVD aluminium Acid zinc-nickel Pass Possible 

Acid zinc-nickel 4 Pass 

Acid zinc-cobalt Pass Observed 

Acid zinc-cobalt 4 Pass Possible 

Table J3; Compatibility of brush plated coating repairs with main coating 

Page 108 



J.6 Figures 

Figure J1; Schematic diagram of test sample used in re-plating experiments 




Page 110 



Distribution list 

Garteur Executive Committee (XC) Members 

Dr.D. Nouailhas ONERA, International Affairs Directorate, BP 72, 

F-92322 Châtillon Cedex, FRANCE 

Mr. W. Riha DLR, Bonn-Oberkassel, Königswinterer Str. 522-524, 

D-53227 Bonn, GERMANY 

Prof. F.J. Abbink 

Mr. P. Garcia 

Samitier 

Mr. B. Uggla 

Mr. A. Amendola 

NLR, PO Box 90502, NL-1006 BM Amsterdam 

THE NETHERLANDS 

INTA Ctra. Torrejon-Ajalvir Km. 4,500, ES-28850 

Torrejon de Ardoz, Madrid, SPAIN 

FMV Defence Material Administration, SE-11588 Stockholm 

SWEDEN 

CIRA S.c.p.A, Via Maiorise, loc. Silvagni, I-81043 Capua 

(CE) 

ITALY 

Dr. O.K. Sodha DERA, Air Systems Sector, Building A8, Room 1005 

Ively Road, Farnborough, Hampshire GU14 0LX 

UNITED KINGDOM 

GARTEUR Secretary 

Mr. E. Maire ONERA, International Affairs Directorate, BP 72 


Members of GoR SM 

Prof. Dr.-Ing. Klaus 

Rohwer 

Chairman GoR SM 

DLR Institute of Structural Mechanics 

Lilienthalplatz 7, D-38108 Braunschweig, GERMANY 

Dr. T. Khan ONERA, Director Materials and Structures, BP 72, F-92322 

Châtillon Cedex, FRANCE 

Prof. R. Ohayon ONERA, Structural Dynamics and Coupled Systems, BP 72 


Dr. M. Mahé Manager Structural Dynamics & Optimisation BTE/CC/MO - 

M0132/3, Aérospatiale Matra Airbus 

316, Route de Bayonne, F-31060 Toulouse Cedex 03, 

FRANCE 

Dr. J. Bühlmeier 

Fairchild Dornier, Dornier Luftfahrt GmbH 

Postfach 1103, D-82230 Wessling, GERMANY 

Dr. M. Gädke DLR, Institut f. Strukturmechanik, Lilienthalplatz 7 

D-38108 Braunschweig, GERMANY 

Mr. H. Schnell DaimlerChrysler Aerospace Airbus, Postfach 107845 



D-28183 Bremen, GERMANY 

Mr. F. Holwerda NLR, Structures and Materials Division, P.O. Box 153 

NL-8300 AD Emmeloord, THE NETHERLANDS 

Dr. J.M. Pintado INTA, Composite Materials Dept. Ctra. Torrejon-Ajalvir Km. 

4,500 

ES-28850 Torrejon de Ardoz, Madrid, SPAIN 

Prof. A. Blom 

Dr. H. Ansell 

Prof. P. Curtis 

Dr. R. Collins 

Mr. M. Fariola 

Aeronautics Division, FFA, FOI Swedish Defence Research 

Agency, SE-172 90 Stockholm, SWEDEN 

Saab AB, Future Products and Technology, 

S-58188 Linköping, SWEDEN 

Techn. Manager Polymers, Composites & Structural Design 

Structural Materials Centre, DERA Farnborough 

Hants GU14 0LX, UNITED KINGDOM 

Fatigue and Damage Tolerance, British Aerospace Airbus 

Filton, Bristol B599 7AR, UNITED KINGDOM 

CIRA, Via Maiorise, I-81043 Capua (CE) 

ITALY 

Members of AG 17 - Cadmium Substitution 

Dr C. J. E. Smith 

Dr K. R. Baldwin 

Mr F. Andrews 

Mr C. Brindle 

Mr R. A. Humphreys 

Mr E. Hultgren 

Mr L. Magnusson 

Mr D. Marchandise 

DERA Farnborough, Hampshire GU14 0LX, UK 

DERA Farnborough, Hampshire GU14 0LX, UK 

Short Brothers plc, P.O. Box 241, Belfast BT3 9DZ, UK 





Aerospatiale, F-92152 Suresnes Cedex, FRANCE 

Mr E. Kock Daimler Benz Aerospace Airbus, D-2800 Bremen 1 

GERMANY 

Mr W. t'Hart 

G Vaessen 

D Wilkes 

NLR (NOP), NL-8300 AD Emmeloord, The NETHERLANDS 

Sergem bv, NL-2289DV Rÿswÿk (ZH), The NETHERLANDS 

ADR(PS) - R(PS)3, Ministry of Defence, Main Building, 

Whitehall, Greater London, SW1A 2HB 

Page 112

Cadmium Substitution - garteur

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